Almost all the decepticons died including megatron, soundwave, starscream, and shockwave. A fourth film would have to be a reboot or else it would be completely lacking half of the important characters of the transformers franchise
I believe its been stated this will be a new start but in the same continuity but it is so far away I think all the info we get now will be subject to change
Who do you want as the main villain for Transformers 4?
Its a bit early to start speculating but a precedent has been set for Megatron returning as Galvatron and then perhaps we could get Unicron, but with the appearance of the Fallen in the movieverse I would not be surprised if it was alot more obscure than that