Bird lost in 5

4. ... g5 has been the main line of From's Gambit for probably at least 150 years. It shouldn't be a surprise.
Black's third move is already threatening to force mate starting with ... Qh4+. White prevented this threat by playing 4. Nf3 but Black reinstated the threat with 4. ... g5.
You cannot prevent 5. ... g4, you can only deal with it.
5. d4 prepares to block the enemy bishop with 5. ... g4 6. Ne5 or the enemy queen with 6. Ng5. The latter is a famous piece sacrifice that is great fun to analyze but is apparently refuted. The line contains several moves for each side that took decades for the chess world to find.
5. g3 protects against mate and prepares a landing spot on h4 for the knight. Black is not doing well in this variation in recent decades. Now he seems to prefer various other fourth moves that are probably no better but at least less well analyzed.