It was quite a big jump, in many, many ways. At release it was more similar than it is now, true, but my main server stayed alive on 2.2.5 for well over a year after it released proper, and I don’t mean the OPERATIONMAPLELEAF thing.
It still took quite a few updates for it to surpass 2 in maps (really only when Canada + and Washington came out) and general gameplay and polish.
I disagree with your last statement, personally. I think because of the greater difference people will stay for shorter. Especially due to 3.0s pisspoor performance and drought of proper servers, I do think 4.0 will rapidly outnumber it. A few holdouts will certainly be there, but I’m confident the RP community will jump ship nearly immediatly as soon as the server configs are ready, and so will the casual PVP playerbase.
Unlike 2.0, I don’t see what possible benefits 3 could have over 4, I am very confident that Nelson learned from the somewhat rushed launch of 3 and will release it when it’s properly ready, hopefully with talking to the modding community as well.
Of course, all of this is just speculation until we see it ourselves, but nonetheless.