Your 5. a4 move looked rather pointless.
You had a good game at that point, with your only real tactical vulnerability being that unguarded Bishop sitting on c4.
You should probably either play 5. d3 (guarding your Bishop and preparing to develop another piece), or 5. Qe2 (again, guarding the Bishop) or just castle.
I played a harder bot than normal, but I have defeated bots with over 1000 rating before like the Bishop Bot from September, the Edith Bot from October, and Bob the Cat from this November, but after playing this game with a bot with a slightly higher rating I just couldn't wrap my mind around it for some reason because of the complexity of it for me? I'll try to improve more!