Why so? 4...Bb6 is just fine- white's most promising line is transposing to a well-known Giuocco Piano line.
what is the best gambit for a development advantage?

Why so? 4...Bb6 is just fine- white's most promising line is transposing to a well-known Giuocco Piano line.
I've always had a soft spot for the Muzio gambit. I don't know if it has survived in the age of engines, but it definitely gives White a development advantage!