Why is the engine eval +1 here? (blackmar-diemer gambit)
Compare the position after 4.b4 to the French or Caro-Kann Advance. In the latter examples, black is prepared to attack the base of the pawn chain and has a way to involve practically all of the pieces in that plan. In the position above, after 4.b4, the c5 push isn't possible. Blacks d7 knight is getting in the way of a piece that can potentially remove a defender of the d4 pawn. Blacks best way to attack the pawn chain might be the f6 push (Though I'm not sure of the viability of that plan).