Queen's Gambit Declined: Marshall Gambit, how to decline it and keep full equality? I wish to play a specific move order of the QGD as Black, but the problem is that White can throw Marshall gambit at me. 

I do not want to play the sharp lines, I need a solid and fully equal position.

I have tried to introduce novelties two times ( I even asked for help here both times ), but both of them failed since I was able to refute them in my home analysis after careful work ( that is why I have deleted the mentioned questions ). Therefore I have given up on trying to improve the opening and am asking for help.

This is the move order I have in mind:

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1.d4 d5 2.c4 e6 3.Nc3 c6 4.e4!?


This is known as Marshall gambit. As I have said earlier, I do not wish to accept it, I need solid quiet positional lines. Something like 4...dxe4 5.Nxe4 Nd7 could work for me, as it is similar to Caro-Kann. The way things stand now, this line is good for White and both of my attempts to improve it have failed, as stated above.

To summarize my questions:


Is there a way for Black to decline the gambit and reach full equality ?
If there is a positive answer to the first question, can you recommend some high quality resources for studying such lines ( books/online resources etc ) ?


NOTE:

I would not mind if in the above mentioned position Black can transpose into another opening, as long as it is solid one.

Thank you for your help. 

Best regards.
I don't know really know the theory here, but I can give some pointers. Maybe they are of some use for you or other readers.

The setup with c6/d5/e6 in the first three moves against white's d4 and c4 is called the Triangle; usually black wants to aim for the Noteboom variation (1.d4 d5 2.c4 c6 3.Nc3 e6 4.Nf3 dxc4 and roll with the queenside pawns).  4.e4 is white's most ambitious and sharp way to attack the Triangle. After 4.e4, you won't see any transpositions to other well-known openings.

If you're not looking for sharp lines, this is a bit of an odd move order to play, because besides the sharp Noteboom there's also this sharp Marshall Gambit to deal with. The Slav and QGD both offer better chances of a solid positional game than the Triangle, I think.

Scherbakov's book The Triangle System won awards and got great reviews; if you're going to play this seriously, that book should be your #1 source.

Black's main option besides accepting the gambit is 4...Bb4; the position looks a bit like a French Winawer (1.e4 e6 2.d4 d5 3.Nc3 Bb4) with the c-pawns on different squares. According to Scherbakov, "White does have a few ways to obtain a small edge, but Black's position remains playable." He also explains that play is rather different from normal Slav positions, and then spends 8 pages discussing it.

I general I suspect that as black, guaranteed quiet play and complete equality is a bit too much to expect in any opening. If you want to dodge white's sharpest options, you're going to have to make some concessions.