As you know, UE4 can make 2d game, but it is only 2d scroll game. I want make a level like ‘RPG Maker’ please see below picture.
I want to combine 2d game and 3d game.
If you have any the information about it, please tell me. 
As you did not state what you like to archieve by combining 2d and 3d, i assume you like the overview similar to the 2d pic but also be able to Switch to 3d. to do this, you just build the 3d and use a camera (and optionally post-effect) from above to get a view similar to the 2d view.
They did this in The Legend Of Zelda - Ocarina Of Time, when Link was in his house in Kokiri Forest. See here at 5:10:
[video]Zelda: Ocarina of Time Playthrough #040, Revisiting Kokiri Forest as an Adult - YouTube . How, I dont know.