So, this is what I always play if I get the chance:

1. e4 e5 2. Nf3 Nc6 3. d4

Now people take the pawn 3. ... exd4
I grab the pawn 4. Nxd4

Now online people will grab that knight 95% of the time 4. ... Nxd4
I take 5. Qxd4

I never get to play a real main line of the scotch because people just grab that knight on move 4. The books say my position is easily winning but it is not always that easy. Do you guys think white has a big edge here? Well he has an edge, but how big?
What is the key strategy in the middle game after the line 1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Nxd4 Nxd4 5. Qxd4 ? Should I play very aggressively or what is the key to maintaining that advantage?

Yeah, you have a big advantage as you have all the space, and your queen is actively placed. Black meanwhile has a hard time developing his pieces. I'd say just develop and keep black from playing ...d5.

Ah ok, thanks for the tip!

No books say the position is easily winning for white after 4. ... Nxd4.

You just have a positional advantage, that's all. Worrying about playing a "main line Scotch" is pointless - you need to understand how to play chess. Study tactics and endgames - how to handle the rest of this opening will become clear enough with that study.

A good example of how meet 4...Nd4 is as follows: 4...Nd4 5.Qd4 Qf6 (5...Nf6 6.e5 Qe7 7.Be2) 6.e5 Qb6 7.Qb6 Qb6 8.Nc3 and White is better.