4...d6
Its passive, but at least u avoid many tricks and transpositions.
4...Nf6 and 4...Bc5 are the main moves but need to be well prepared, many sharp variations and also positionnal which are not so easy to handle.
With d6, u can continue with Be7, Nf6, 0-0.
White is better (maybe?) but not so much and u succeeded getting your king safe.
Some considerations :
- If white wants to continue in gambit mode with 5.c3, then u can simply take and continue your development. The fact that your bishop is on e7 avoids many tricks (Bc5 or Bb4 can be used by white in some variations), and u avoid dangerous pining move Bg5. U also have something like 4...d6 5.c3 Ce5 first and then dxc3 but no need for this.
- Once u play 0-0, play usually continue with putting your rook on open file, so e8, u could attack e4, and white has to take care about some d5 possibilities for black.
- U can play in Pirc mode with g6-Bg7 instead of Be7, but more complicated, u asked a simple system, so i propose Be7. 
Here are some games :
This opening keeps on schwaping me. Does anyone have any solid lines to play as black. I don't want to go into anything too crazy. Below is the typical line I see at the start of the gambit.