If you open d4...

, all textbook openings aside,

and black neither attacks d4 nor covers or is in an attacking position on e4, would you automatically play e4?

Bad OP. Please rephrase, I do not understand.

most of the times, yes

Therobz and Cuendillar are right.

It's a perfect fine play and White is certainly welcome to; however, it's not automatic that White would play 2. e4. The reasoning behind this is that unless Black plays something completely random like 1. a6, his opening move will probably attack e5 or d5 if not e4 or d4. For example, if Black plays 1. e6 and White plays 2. e4, then Black to immediately transpose into the French or play Nf6 and transpose other openings.

Without the proper support from other pawns, like c6, or other pieces, like Nf3 and Nc3, the e4 and d4 pawns are vulnerable by themselves. Ultimately White wants to be able to control the center with e4 and d4, but it usually takes some preparation because White can't extend his line too far otherwise the Black pieces, which are closer to the action, will overwhelm White's initial offense.

But realistically why White can't play 2. e4 right away is because Black's first move almost always attacks either e4 or d4! Like cuendillar, I personally like 1. Nf6 in response to d4 :)

OK,

  so, if you're nt going to play a pawn on e4 you at least want to defend it to have a strong centre.  This is fair to say.

Yes, that is what I meant, a play like a6, or more specifically 1...e6, 1...d6, or 1...g6, in terms of popularity.

I'm glad cuendillar raised the point of transposition.  I think that this is a tough idea for a beginner to wrap his head around, that openings can be reached through different sequences.

If 1.d4 e6 was followed by 2. Nf3, you would likely see 2...Nf6 3. c4 d5  for the Queen's Gambit declined. Or 3...b6 for Queen's Indian. 

I know I would.
Dont really like openings anyway, and the sooner I develop my pieces and castle, the better I think it is.
So if he's giving away central control, I'd be more than glad to have my d and e pawns on the 5th rank.
I'd then most likely defend them with my knights, followed by the bishops and c pawn.