Also another early thing was with Tristan is his teething, first one broke through also at 4 moths. Well Arwel will be earlier in that I'm sure, he's got all rosy cheeks and you can see the teeth getting whiter and whiter.
We can see that he will be an active one, not a slow one like Elen (who also caught up since).
Here is a video of Arwel being hyper. he just wants to go :)