I have a problem with this move order because I can neither play my favorite Botvinnik Variation nor a Meran with Whites Bishop locked in at c1. What am I supposed to do against this move order.

Don't play 4...e6. That just plays into white's hands.  Play 4...dxc4 and be happy you were able to grab the pawn.

lol

But then white plays 5.a4

I agree with ghost of maroczy, dxc4 is the mainline isn't it? after a4, play Qb6.

Vonzi, why are you afraid of 5 a4?  Play 5...Bf5.  Things like ...dxc4 and ...Bf5 should be natural to a Slav or SemiSlav player.

4. ... Ne4

Maybe you can also consider changing your move-order and play 3...e6, unless there is some unsuspected drawback here

Then he would have to learn 3. ... e6 4. e4 among other things.

There's nothing wrong with his move order at move 3 and 4. Bg5 is nothing to fear because of 4. ... Ne4 and black is going to get a good game.