Classic Ironman An Army Of Four (EU2012)

Preparations

Enemy Within & Slingshot

The An Army Of Four achievement is only available with the Enemy Within DLC. The Slingshot DLC gives the following advantages: More experience, one key additional plasma rifle, a medium high-Aim Heavy soldier (which can fill a hole and be a backup unit on Base Defense), easy massive cash infusion, and easy Fusion Core loot (otherwise only available when shooting down Battleships late-game). Disadvantages are one or two tricky missions which must be handled carefully and must be prepared for with either impeccable game-play, luck, or very specific soldier abilities that make them much easier.

Operation Progeny gives extra loot, experience and several guaranteed psi-soldiers.

Tutorial Options & Second Wave

The basic Tutorial gives you some advantages. The disadvantages is that you cannot skip the cut-scenes, it makes re-rolling your first soldiers and starting base configuration extremely time-consuming. If you've gotten to the point where you're even considering a Classic or 'An Army Of Four' Ironman attempt, then you likely have little patience for baby step-by-baby step instructions.

The Meld Tutorial is extremely advantageous, on the border to outright cheating, and should always be taken. It's extremely fast, gives two easy Meld pickups and makes the first mission carry almost zero risk, given that several alien pods (hereafter referred to as 'pulls') are single-alien groups.

New Economy makes it easy to increase the first month's income.

Not Created Equally makes it easy to get at least one 80-Aim soldier on the first mission. Focusing on leveling a few 80-Aim soldiers will drastically affect the use and deadliness of Heavy class soldiers, which otherwise suffer from low Offensive stat increases.

Training Roulette makes soldier skills more unpredictable but the game exponentially more replayable. Highly recommended for the returning player.

Aiming Angles takes away some of the cartoonish boardgame-style of the original game in favour of a more realistic simulation of actual tactics. However, it does not much affect the difficulty of the game, though it may make it slightly easier.

Re-rolling first mission

Ideally you will have two soldiers with 80 Aim with Not Created Equally, for a significant chance of one of them being a Heavy. Two soldiers with 75 Aim is an easier target, given that you will have options for the second mission.

Ideally you will have the first or second square to the right of the first-level Access Lift open, in order to get a Workshop up quickly. This significantly affects the ease of getting money and facilities done on or ahead of schedule the first month, and is worth re-rolling for, even if you rolled two 80-Aim soldiers.

Re-rolling a few times you will quickly get a routine for which boxes to check when starting the game.

Knowledge

It is imperative that the player is fully cognizant of various bugs and intricate quirks of the game. For example, Improved Pistol 1 and 2 are known not to have an effect as of January 2014, other than possibly Imp Pistol 1 (and 3) for a Covert Operative. Some escort missions, the first two Operation Progeny missions and the Confounding Light Slingshot mission can be absolute murder unless you remember where and what triggers when the aliens drop in. A simple thing like forgetting that an alien does not trigger Overwatch fire when it moves 'into' a square, only when it moves 'out of' a square (including dropping in from above, and the Floater's Launch ability) can easily turn a simple situation into a life-and-death struggle.

Africa is generally seen as the easiest starting continent, as it makes the first months slightly more forgiving.

Satellite Rush Strategy 1st month

For further discussion see Survival Guide.

Day 1 - Excavate ($10, 5d, space for Workshop);
Days 4-7 - Abduction Site (+4 engineers, for Workshop), occasionally there is an early Council mission;
Days 7 - Workshop ($130, 10d), excavate ($10, 5d, space for Power Generator);
Day 8 - If the first Abduction Site comes too late to get a Workshop-to-Uplink chain first month, then your options are to reroll or have the rest of the game play a step harder.
Days 7-10 - UFO (small scout, 1 interceptor needed). Let the first UFO go, shortly after you get a Medium Scout with more loot and experience.
Day 11 - 3x-4x satellites ($77 each with 9 engineers; 20d) not in pack (1 by 1). If there was no Council Mission or Medium Scout by now, your options are to reroll or persevere. Approximate 10% probability.
Day 12 - Power Generator ($60, 5d, power for Satellite Uplink);
Day 17 - Satellite Uplink ($100, 14d);
...
Day 31 - Launch satellites for maximum cash with an eye to getting the South American continent bonus as soon as possible.
Council Report

Research Order

Genetic modification for Mimetic Skin quickly makes things so much easier it's silly, and can dramatically improve the odds of one experienced and modded squad leader being able to help 80-Aim rookies through their first mission or two in the middle game. Be aware that the Close Combat Specialist with Mimetic Skin is in for a surprise, while the soldier with Low Profile and Mimetic Skin is nearly always invisible. Bioelectric Skin in comparison is only needed on one single soldier. How much you want to abuse Mimetic Skin is up to you.

Alien Containment and the Arc Thrower dramatically cut down on research times with the South American continent bonus.

Carapace Armour dramatically improves soldier survivability.

Gearing

Laser Rifles are generally cheap and quickly gotten, ideal for your Support and Assault classes.

One MediKit will generally suffice, but with a four-person squad you will find that a dedicated medic is a must, with either the Deep Pockets or Field Medic abilities.

S.C.O.P.E.s tend to be extremely useful on all but Assault class soldiers, given that with only four soldiers you simply cannot afford to miss shots.

Tactical Rigging is a nearly game-breaking change that dramatically simplifies gearing choices and makes your squad much more durable and powerful.

The Mimic Beacon can often save one of your soldiers from an otherwise certain death. Other items are generally not needed for the extremely careful player, and will delay more satellites early on.

Tactics

Retreat from contact and make sure that the aliens take hits from your Overwatch soldiers. Pulling a new pod of aliens before you're done with the current pull can quickly doom your squad, and a few dead soldiers or one aborted mission immediately makes all following missions harder. That said, quickly killing a new pull can often minimize the risk faced.

Terror Missions generally demand extremely conservative, cold-hearted play. Going for perfect rescues is not recommended, the risk is simply too great. More zombies also give more experience, unfortunately.

Many missions benefit from abusing the AI's focus on the last/nearest enemy seen. This means that many waves on the Base Defense mission or the second Slingshot mission can be delayed by one single soldier with Mimetic Skin popping in and out of sight.

Sectopods

Plan for this, or you will find yourself accidentally taking down the Overseer UFO early on, without sufficient firepower to overcome the new Reinforced Armour ability in one single turn, meaning your four soldiers will take heavy damage over multiple turns. It may be worth delaying this fight until you are absolutely certain you have a way to deal with the Sectopod. The alternative is a long game of hide-and-seek with dubious prospects in a very dangerous unknowable tactical situation.

Known Difficult Missions

First Council Mission - the Sectoids on the far side become very simple to take out with a Squadsight sniper. If you lose personnel here, consider rerolling.

Base Defense - you need to know the order of the waves. Do not summarily dismiss an early Heavy squaddie with Aim score of 52, as they can be useful as a reinforcement here.

Confounding Light - Sprinter, Squadsight, Mimetic Skin and Lightning Reflexes all make this mission dramatically simpler.