If black 'mirrors' whites 4. d4 in the Scotch 4 Knights and white wants to play sharp, aggressive moves. what would be the best move? I know that chess engines like stockfish, GNU Chess etc. would calculate the next move for white as Bb5, but I want to exchange pieces. So what would the strongest capture be here?
According to https://www.365chess.com/opening.php?m=9&n=17985&ms=e4.e5.Nf3.Nc6.Nc3.Nf6.d4.d5, exd5 is the strongest capture for white, with a 83,3% win rate.
Here’s a site PDF that I found that goes really in depth on the Scotch Game and other variations of opening play: https://terrycucf.weebly.com/uploads/1/9/2/9/19295043/various3rd.pdf
I hope that it helps you!
4...d5 is really bad, if nothing else it leads to at least a clear pawn up for white in various lines that can ensue and they all involve frequent trades as well (as you desired), here are a few options starting from
Nxe5 (a choice well in line with your preferences) leaning on more aggression/direct play and less positional compared to some of the other choices (like your suggested move):
[fen "r1bqkb1r/ppp2ppp/2n2n2/3pp3/3PP3/2N2N2/PPP2PPP/R1BQKB1R w KQkq - 0 5"] [title "5. Nxe5"] 5. Nxe5 Nxe4 (5... dxe4 6. Bb5 Bd7 7. Nxd7 Qxd7 8. d5 a6 9. dxc6 Qxd1+ 10. Kxd1 O-O-O+ 11. Ke2 axb5 12. cxb7+ Kxb7 13. Nxb5) 6. Qh5 Be6 (6... Nxe5 7. Qxe5+ Be6 8. Nxe4 dxe4 9. Qxe4) 7. Bb5 Nxc3 8. bxc3 g6 9. Nxg6 fxg6 10. Qe5 Rg8 11. Qxe6+ Qe7 12. Qxe7+ Bxe7 13. O-O
Note: Bear in mind that by intentionally opting for lines such as above where you favour trades and direct aggression, you'd be naturally walking into highly tactical positions, so be prepared to calculate many many relevant lines at each point (specially in the given examples above).