After a minor course correction I investigated forms for the adult and in some instances juvenile Four. For this, I looked at sphinx cats, naked mole rats and babies. I'd like to keep the mouth, inspired by the hydrothermal worm, because I think it makes the creature look a bit more alien, and could influence the reveal of the eyes. As before it opens its eyes it'll be puffy, pink, generally unsettling just moments before David realises it has a beauty to it.
I might switch out some features as I don't want this to look too much like a rat. I might tweak the ears so they could be embedded on the side of the head. From discussion, this creature will get less saggy as it matures and grows into its skin
I used image referencing to try and paint what Four's skin could look like. It's rough but I plan to do more refined concepts later which hopefully should properly convey the creature's appearance in regards to whether it's cute or grotesque. This sort of drawing also makes it easier to show if it's translucent or not.

After discussing with Phil there could be a beauty to the creature. The above sketches focus on movement because that's where I think this stage of the creature will come to life the most. This creature evolved in a marshy environment, it would likely be a hunter that could bound off trees or sprint through hills. This might be contrary to its previous forms which are a lot more bulky and cumbersone. But one thing that Phil and I discussed was that this creature had to be built as a survivor. Four is established as a hunter as in its grub state it eats moss and small insects. So in its adult form it could have evolved to hunt something larger, which requires it to be a bit more of an ace than the flabby thing that would emerge from the cocoon.