For invading 4/4 bases:
Use a silenced sniper rifle that can one-shot guards. The American semi-auto sniper rifle with an assault rifle magazine is good for this.
Use the side catwalks as much as possible. You can kill a guard in the middle of platforms and it will attract the attention of the guards while you slip by on the sides. Get past the first three platforms as fast as possible. When you're on the bridge going to the last platform, take out the UAV's, then snipe some of the guards on the platform. Make a beeline for the core door, but make sure to not set of any mines. Murder all of the guards you come across, and abuse reflex mode if you need to. You can also get attention away from yourself by throwing decoys or making explosions away from your position.
Basically, skip as much of the first three platforms as possible and silently murder the last platform.
Edit: Try to not do any actions that have a long animation (hanging, jumping on top of a container, ladder climbing, etc.) unless you know a guard won't spot you. If you get spotted in the middle of one of those, your reflex mode probably won't last long enough for the animation to finish, and the whole base will go on combat alert.