On the outside these guns are almost identical, but on the inside they are totally different. The Bolt B4 SOPMOD is a conventional battery powered AEG with a near normal V2 gearbox. However the heavily reinforced Bolt gearbox also includes a push rod that extends through the rear of the gear box and moves a large weight backwards against a spring in the stock tube. When the trigger is released the weight flies forwards creating a recoil effect. The recoil is felt as a slightly soft, but well felt push backwards. On a standard Bolt the weight is then arrested by a buffer spring. As it the recoil weight sits between two springs it then wobbles back and forwards a bit and a “boing” sound can be heard. This is very unrealistic. However if you replace the buffer spring with a stack of silicone O rings you can eliminate this annoying “bounce” and the recoil feels much more realistic.
The WE M4 is a classic gas blowback gun that is designed to run on green gas (a type of propane). The gun has a bolt just like a real gun that in this case is propelled backwards by gas pressure when you press the trigger. This tend to make the gun move forwards in response to the bolt moving backwards. Once the bb has been fired the bolt springs forwards under spring pressure (creating a rearward jolt) and slams into the hopup chamber providing a third jolt. The recoil is felt as a sharp jolt that comes the instant the trigger is pulled. With a standard gun there is a lot more recoil from this gun than there is from the Bolt B4 and if you want you can get heavier bolts and buffers for more recoil. But being a gas gun, while you get near instant trigger response and great sound you do suffer from cool down effects as the propane cools when you fire rapidly. In addition the mags only hold around just over 30rds.
The Tippmann M4 is a purpose designed air powered airsoft gun based on Tippmann’s highly regarded paintball guns. You can use either 80 round CO2 magazines that take a single use 12g C02 bulb, or you can run it off an external air or CO2 cylinder and then use regular AEG magazines. Like the Bolt AEG recoil comes from a weight in the buffer tube that is pushed backwards by the guns air bolt. Again the recoil primarily pulls forwards and again like the Bolt B4 there is a lot of bounce and a “twang” spring noise. The feel of the recoil is similar to the Bolt B4, slightly soft but readily felt, although I felt the Tippmann had less recoil than the Bolt B4. When running off the CO2 mags there is some cool down and reduction in power from the first shot to the last. I got 400fps on the first shot which dropped to under 300fps by the last. On external air there is no cool down and the fps is very consistent. You have to change a few (included) parts inside the gun to switch between CO2 cylinders and external CO2/Air, so you can’t easily swap mid game.
In terms of realism the WE M4 is for me the most realistic. The recoil is sharp and distinct. There’s no obvious spring twang or wobble. The guns bolt operates very much like the real thing. The downsides are the limited mag capacity (although you also get bolt lock back when the mag is out adding to the realism), cool down loss of power when firing rapidly or trying to use the gun in very cold temperatures. For convenience the Bolt B4 is easy to use. It takes standard AEG mags so you can use Hi-Caps, Mid-Caps or whatever takes your fancy. It’s not bothered by cold weather and you don’t need an air tank on your back. But the recoil is less than the WE and it has that annoying sewing machine like whirr that all AEG’s produce. The Tippmann is a bit of a wild card. It sounds a lot more realistic, no sewing machine whirr, just a sharp “phat” as the BB leaves the barrel. The recoil is a bit soft and bouncy in the stock gun but there are aftermarket heavy bolts to increase the kick. But with the CO2 cylinders you’ll need a CO2 capsule for each 80 round mag and have to live with big changes in hop and velocity as you go through the mag. Or you carry a bulky external cylinder and air line, although well though out air rigs are barely noticeable during a game. Plus you’ll need to have somewhere that can fill your tanks for you.
There’s no one clear winner out of these 3. If you simply want the best realism and snappy trigger response then I’d go for the WE M4. If you want simplicity and ease of use, the Bolt B4. Want the snappy response of gas but with bigger mags then go for the Tippmann.
There is actually a 4th option: An HPA conversion for the WE M4. This gives the same recoil and snappy trigger response along with the ability to use higher capacity magazines! If you use an HPA magazine for the WE then you can switch between HPA and regular green gas just by swapping mags.