How to ugrade any hero in level 4. I'm stuck at this part of my quest pls help?

Your heros upgrade levels with Experience points (exp). If it's a combat hero then you need to assign them to a formation and attack zombies. Each fight will get them some exp. Development heros can get exp from some skills / when stationed.

You can also earn Hero Exp Items when using recruitment tickets and some other quests that you can then use individually to level up any hero you want.