When black refuses the gambit a good idea is play d5, because after this push black blocks the diagonal to attack f7 and white is forced to take the pawn and we will reach a position in which has a strong center but with an isolated pawn
About this variation , I share a game played by Velimirovic in 1974 against Toth, I think the endgame is very interesting
In the fifth video of the series, we see what to do when black castle queen’s side.
In the sixth video let’s see what do against Bb4
In the seventh video we focus the attention about the position
Another way to refuse the gambit is play 4..Nf6.
About it I made the video #8 and #9
Another option that I considered is 4..d3