whats the merit in 3betting A4o pre? seems quite random

You could probably throw JJ, A3s in there too and 89, T9s of clubs/hearts, A7s, 78s too.
Some AQ if he thinks you're going crazy. Which it looks like.

Would pretty much always flat pre BvB. Only situation I can think of doing this is vs someone who is folding a lot to 3bets here, but then still I think we can pick much better hands/bluffs than this.

I do think 3barrelling can be quite effective in this spot against the right opponents, but with that turnsizing you kinda encourage him to play xSh or fold and it's not consistent with how I would want to play my range here. If you want to play exploitable with a high turn frequency I think a 1/3 flop cbet works more effective, followed up with a large turnbet. As played, I think he might be shoving hands like AJ, 88-TT (maybe 7x) vs that sizing now where he just puts you on either a draw, AK or a random spazz. He might also think he has a decent amount of FE if he was observant and saw you make similar aggressive, exploitative plays.

I think Axo plays better in position in a single raised pot bvb. Unless villain folds a lot to big blind's 3bet, 3betting this hand often gets called by dominating hands. And being offsuit cuts off your potential outs from bdfds.