First off, databases say nothing about best move. Stats can often be mis-leading. After 1.e4, the move 1...c5 is not the strongest simply because it scored the highest. In theory, 1...e5 is still Black's best move from a theoretical perspective. Theory and Application are not the same thing. Database Stats are determined by Application, not Theory.
That said, 4.Nf3 has always been well known to be the best from a theoretical perspective because Black can immediately go after the center against 4.e4?! via 4...Nf6 5.Nc3 e5! and 6.dxe5 is horrible and White has lost all advantage after 6...Qxd1+ and 7...Ng4.
White should not play e4 until Black commits to ...e6, where then countering with a central break of ...e5 loses Black a tempo.
I know that the Queen's Gambit Declined, Marshall Defense is unsound, but I never understood the purpose of 4. Nf3, which is the best move according to the database on chess.com. My instinct tells me that the best line would be 1.d4 d5 2. c4 Nf6 3. cxd5 Nxd5 4. e4, attacking the knight while building an imposing pawn center. Why is 4. Nf3 better than this?