Russian 3 KNights Game

Anything wrong with white's 6th move here?

What's blacks best reponse?  

1.e4 e5 2.Nf3 Nf6 3.Nc3 d5 4.exd5 Nxd5 5.Nxd5 Qxd5 6.c4 

Although Game Explorer gives only d3, d4 & c3 in this position, it doesn't mean there's anything wrong with c4.

I don't know about Black's best response but ..Qe4+ looks reasonable.

Thanks for the response Stephen.  That was my first thought too, but then he just blocks with his bishop, and I've got my Q hanging out there to be hastled.  If I go Bb4 to pin his hastling d pawn, he can back my bishop up with 2 pawn moves, then I'll have to move it further down, then he CAN move his d pawn to attack my Q, then he's more developed and I'm running my Q around.  

    Since there never was this last c4 move in any exporers, I thought there MUST be something bad wrong with it.

Any other thoughts?

I only came across this webpage earlier today but when I ran the position through it just now, two of the engines gave Qe6 as best continuations & the third (Toga) gave Qe4, so it can't be too bad.

It doesn't develop, blocks white's own bishop and leaves the d-pawn backwards. 5.Nxd5 must also be wrong, it just develops black's queen for free.

Why not 5.Nxe5? Doesn't that just refute 3...d5?