Having seen the Infinity War footage at D23 and Comic-Con, Groot did look very similar to the way he looks in the post-credits scene...

James Gunn: He's exactly the same.

He is exactly the same.

James Gunn: That's exactly the Groot we designed for Infinity War.

So then that scene is set four years after...

James Gunn: Well, basically those are at the same time.