Episode 4 starts out with Sheldon telling Leonard how he's no longer going to work on time travel because he realized that if he did invent it, he'd just go back in time and give the time machine to himself.
BUT, at the end of season 3 (I don't think the season finale, but one of the last ones of that season), Leonard is sitting in Penny's apartment telling her about when he first met Sheldon. In the flashback, they had an agreement saying that if either of them were to invent a time machine, they would agree to go back in time precisely five seconds after Leonard signs that part of the agreement.
Edited for "Holy run-on sentence, Batman!"
That's not really an inconsistency. It's really splitting hairs, but it can still be congruent. It just means
they didn't invent a time machine (that can travel backwards, it is actually much easier to travel forwards in time) or that Leonard is a liar. It wouldn't be the first time he broke the contract. He could still go on to create a backwards capable time machine AND not show up in the past as per the contract.
Sheldon could still work on it, even without just taking the contract at face value. I mean I want a mansion and a Ferrari and all kinds of ***, I can still hope and dream my *** off and think someday, somehow I'll obtain it.
All that aside, it's not a necessary inconsistency for Sheldon to have this revelation. The statement refers to his solo work on the time machine, whereas the contract entailed the both of them.
Like I said, splitting hairs, but when you start dealing with temporal paradoxes and ***, everything comes down to splitting hairs and word by word specifics.
Also BBT is awesome.