CR 4 does seem about right. The Advancement appeals as well.
Updated.
Finished?
Afraid not.
Mechanically, the Swallow Whole has "xdx bludgeoning and x acid".
Our
Nereis conversion did 1d4 crushing plus 1 acid but was Medium sized.
Increasing that to Huge suggests 1d8+9 plus 4 acid.
That seems about right for a CR4 monster, although I don't object to increasing it to 1d8+13 or 2d6+9 for the bludgeoning or 6 points of acid damage.
Methinks the description and flavour could do with some work...
A long thick worm erupts from the sand, presenting a blank, roughly spherical head covered with short spines. An eversible proboscis extends from the head, seeking prey.
There should be a comma after "long". Also, how can the viewer tell it's "long" when only its head is sticking out of the sand? Also, the proboscis would be the first thing you see, since the worm shoots it out at prey.
An ottoia is a carnivorous primitive worm found mostly burrowing in sandy reef bottoms.
Wouldn't "primitive carnivorous" be better, I also feel "mostly burrowing" is clumsily phrased.
An ottoia lies in ambush, waiting to detect approaching prey via tremsorsense. It generally fights with only his its head exposed, gaining both cover and the benefit of its armored head.
Apart from the "it" generally fights with "his" and the "tremsorsense" typo I think the wording could do with more polish as well as more info on how it actually fights.
How's this:
A long proboscis erupts from the concealed burrow of a massive worm as thick as a barrel. The worm's blank, roughly spherical head is covered with short spines. Its eversible proboscis quests about in search of prey.
A ottoia is a primitive carnivorous worm that lives in burrows in the sea floor. They are mostly found in sandy reef bottoms.
The body of an ottoia is 2 to 3 feet in diameter and 30 to 50 feet in length, weighing about 5,000 pounds.
COMBAT
A giant ottoia lies in ambush, waiting to detect approaching prey via tremsorsense. It attacks as soon as a creature small enough to swallow whole come within its reach. An ottoia generally fights with only its head exposed, gaining both cover and the benefit of its armored head.