1.4 contains lots of stuff. One big thing is that many of the bugs that currently exist will no longer exist. Mobs, minecarts, item drops, arrows, snowballs, etc will work correctly as well because there will be actual physics programmed into it. Along with that, the plugin API is being changed, so all the current plugins will break, but it will allow new plugins to do much cooler things! Also, the server itself will run more efficiently and faster, and it will work with some of the new multiplayer features added to 0.9.0 (such as worlds now sending chunk information compressed, making it faster and easier to send to the player, with less errors). 1.4 will also support all the things in 0.9.0, and be prepared for future MCPE updates, such as redstone and enchanting.