This was already implemented in Toribash versions a long time ago.
There was a simple way to get 4 players in a replay, thus letting you make 4 player spars in SP.
It was removed due to glitches, and lag. The amount of times this has been suggested is astonishing. I see literally less then every 2 weeks, a thread like this. Anytime there's an update or people see hampa in-game, they also suggest it. Stop friggin suggesting this. It was removed for a reason. I'll explain why, in the hopes that you people who don't know jack-shit about computers other then how to get a virus, watch **** and play video games.
The amount of RAM Toribash uses with just 2 people is a large amount. The fact that it's a 3D game, requires more RAM then 2D games. Then the scripting takes another chunk our of it. You probably don't know but scripting a game where you move every joint requires basically the following. Code generating the movement of each joint, maximum direction, speed e.t.c., then you've got code that determines the amount of force which is required for a dismemberment/fracture. Since you can put dismemberment/fracture threshold where ever you want, that requires an algorithim, substituting certain numbers (dismemberment/fracture) into an equation. Then you've got the code which actually governs the physics of the game (friction, gravity, momentum, e.t.c.). That's about the basics of it, I'm sure that there's more in there then what I know. After that you've got the graphics. 3D graphics, realistic or not are 3D. That requires a graphics card that supports 3D. Generating the models, textures and whatnot also takes up RAM. That's in general the amount of RAM required for 2 players.
Let's assume that your computer has 8GB of RAM. There's a little bit of it that disappears starting up your computer. Let's now decide you have 7.8GB of RAM. On average on my laptop, when I start Toribash right away I go down to 6GB approx. That's with 2 players. Not even clicking anything, just the codes governing scripts and what not. Assuming you started to play, you'd lose some more RAM as the code is varying. You will not do the exact same thing each time, thus forcing your RAM to decrease slowly each time you have to change something, using some code here or there, which takes up that little bit of RAM. It may not be noticeable at first, but if you're like me and you only hibernate, after a couple days your computer is running slower then the fat guy in a marathon. Imagine the effects of Toribash+whatever other programs you're using (Google Chrome, Adobe Photoshop CS6, e.t.c.). Double the effects of Toribash on your system, and boom that's double the amount of RAM used up (In Theory). Don't forget I only listed some things. There are others, the replays for example, the computer has to fetch the data using Random-Access Memory which is faster.
Now let's assume you don't give a dam about anything I just said and said "I just want to play 4-player with my friends so we don't have to wait to fight each other" or "It'll look so cool, every one will want to suck my nuts". Doing this online would require your modem to send 3 times the amount of data it normally has to send, and receive 3 times the amount also. That's all fine and dandy when you have your Hi-Speed Unlimited Wi-Fi. When you're like me and you're at your cottage on the weekends stealing your internet from your neighbor in the middle of a forest, it's slow. But I can still play online owing to the fact that the demands on the internet, while not overly small, and still handle able. Tripling that puts strain on people who just want to have some fun, and have slow internet.
Last part, why hampa isn't going to change the traffic/replays to binary. First off that would change the game engine, and as many mods have said there will be no current changes to the game engine. Second all replays, past and present will be broken. Thousands of Thousands of peoples hours of work would be lost, just because of that. Third there's no point, the replay files aren't even that big. The pre-installed ones (there are a SHIT TON) don't even take up 30mb. That's nothing. Keeping that loose gives both better quality and fetching. Fourth, any ability to modify replays would be gone, unless you knew Binary, and the average 14 year-old doesn't know it. When you make a spar, save it, and mess up your pose, there's so many ways to fix it.
Edit it, re-save it, and then change the name of you opponent from Uke to whatever it was before.
In Total Finishing of this explanation, you want to play with 4-players?DOWNLOAD TORIBASH 3.93.(I think.)