First of all if what's written here sounds stupid to you, I'm sorry about it but I have no idea how all this connect with each other.

I don't know what's keeping us from using the keyboard/mouse combo on PS3 but I know it does (eventhough it's possible but with lags or something like that) so I was wondering if the keyboard/mouse combo could work on PS4 since it's similar to a PC or is there no link between these things at all?

That's it. (Just saw that the developper of Total War supported the PS4, so that popped in mah head)

you can play dust 514 and a few other games with keyboard and mouse

Every USB keyboard/mouse works on PS3.

Just the devs blocked it use in 90% of the games.

Sony never restricted the use of K&M for games (Unreal Tournament 3 uses it for gameplay on PS3) , but devs didn't want to introduce these into their games, because K&M players in multiplayer FPS/TPS and RTS games have a big, big advantage over pad players and might therefore sour the experience for the majority.

And devs probably didn't want to spent time to balance single player for this input method either.

They will probably allow it just like with PS3, except this time more devs will be interested in the platform. With the trouchpad on the DS4 devs who otherwise would be hesitant making a game that heavily requires a mouse might make PS4 games as every user will at least have some sort of cursor equipment to play the game.

PS4 should definetly support mouse and keyboard.

shouldn't be to hard. doesn't sound impossible.