Why I can't attack level 4 zombie?

If it is not letting you at all then in some cases you need to have attacked each zombie level in turn (1, 2, 3, 4 etc).

If it is telling you they are too strong then you will need to increase your combat strength. Level 4 zombies are a large step up from level 3. (I don't know why, but they are).

Train more troops, and upgrade your barracks to increase the numbers you can march in the formation. But also use the research options in the Institute under Basic combat and the vehicle, shooter and fighter training sections to increase their effectiveness. This will boost their power and let you fight the higher level zombies.