What should a beginner play e4 or d4?

I can tell you what a master said once.  d4.  His reasoning behind this decision was based on his premise that it's hard to defend the pawn on e4, or it's not always straightforward to defend it.  In the case of the Kings Gambit, and the Latvian gambit, I can see where this is coming from, however, beginners are often advised not to play gambits, and when a beginner does play a gambit, it is because they booked up on the position and know all the traps.  This is bad because it's commonly known that a beginner shouldn't spend a whole lot of time on openings.  Another try for black to undermine the pawn on e4 is to simply play the nimzovisch variation of the opening as follows:

Just note that black is not entirely won in the opening.  he has to fight, and chess is a fight.  There are various traps yes, but it's pointless to learn them because white has a better way to defend.

Of course these moves are hard to find for beginners, however, chess is a game of learning.  you lose a game, then you look up the opening, and find out where you went wrong, then fix the problem, memorize the better move, drill it, and go at it again.  That's how you learn, from making mistakes.

I just got to say this, a master told me to not play gambits.  Get to 2000 first, then play them.  That's the advice given to me, so I don't know if the beginner reading this will listen or not, just know, that you get more bang for your buck going over master games then by studying openings because you never know what exactly your opponent will throw at you, he may take you out of book, what will you do then?  I can tell you I have been consistently playing an improving beginner and he lost a lot to me because I took him out of book.  It was like he didn't know how to play chess when I took him out of book.  Instead of studying openings, improve your general chess skills by analyzing your own games by yourself first, then with a stronger player, and take notes.  Look up your openings after you play bad openings 9 times out of 10 the answer is just a database away. 

In my opinion, it's harder to play the black side of e4 e5 as a beginner than it is for white.  Here is why.  Most beginners, know absolutely nothing, obviously, especially if it is the first game.  Lets say someone told them to play e5 after white plays e4 and go from there.  White plays Nf3.  Now it's blacks turn.  What to do???  Lets say this is blacks first game, and has no idea about anything.  A common sense move to play would be Nc6, protecting the pawn, while developing a piece.  But, black is a beginner, and doesn't know that he has to develop his pieces, all he is going to do is play the first move he thinks looks good.  Let's say he plays Nf6, he decides to copy his opponent.  Now white plays Nxe5.  He didn't even consider that his pawn could be taken, and because he is a beginner, defends horribly, and loses his queen.  This can easily happen at the beginner level.  All it takes for white to know the motif of the double check on the trap that wins a queen for a knight.  It can easily happen, especially if white has played some games and fell prey to it before, and "learned."  However, Chess is a learning experience, and decides next time he will play d6 to defend the pawn.  5 games later though, after he wins 5 games as black with d6 he wants to be different, and plays f6....????  you know the end of that story.  It isn't until you try different things and lose to them until you learn not to repeat your mistakes.  In order to get good, you got to lose.  I think starting out with openings like the french or the carokan as black is a mistake because you don't learn how to directly oppose a pawn center, which is a useful skill to know.  Of course you can still play the french or the carokan from time to to time, just play e5 sometimes, or most of the time, or whatever, or never, I don't care just play something.  Forget about learning to directly oppose the pawn center!  Do what you want.  My two cents, just take it as that.

I just think it's an easier strategy to learn how to attack the f7 square as white, than rather playing the queens gambit, or the london system.  In the case of the queens gambit, the beginner is hard pressed to get anywhere unless they memorize a bunch of lines because in the queens gambit opening has the most variations I believe than any other opening in the game.  If you play the queens gambit as a beginner, and play in a tournment, I wouldn't be surprised if you are taken out of the book on the 4th move.  That is just my experience talking when it comes to the queens gambit.  But hey some people like memorizing a bunch of openings so heck why argue right???  Again do what you want.  Also, most black players often don't know what they are doing in response to the queens gambit, and often create a pet line to try and keep the gift in the opening, little did they know, that's what a good queen's gambit player wants...  So the queens gambit can be good in the right hands, someone who's played chess for awhile, and moved over to it, like me.  But, in my opinion, the concepts are a lot harder to understand for a beginner.  It's easier to just play e4 and attack f7 tbh.  In my opinion, that is.. don't forget that .

In the case of the london system I just got to say system based openings arn't good for beginners, because they don't learn how to make the most out of every move, and often have to spend time making waiting moves, which is not conductive of playing to maximize the potential of every move, which is what you really should be learning in chess.  That being said, the london system is a good opening.  I'm not dising the opening, just saying, not good for beginners.  Again that is my opinion.

That is my lowly opinion.  Take it or leave it.