Need some help making a Werecroc

[Hypersmith]

I got a lab making lycanthropes in my game, which my players are currently trapped in. There are two days to the full moon, and they have a three day window to escape. I wanted to make one of their fellow prisoners a werecroc, and the stats I have in mind are basically giant crocodile stats, with a few changes:

18 strength, and normal intelligence/wisdom (10 or so) and lower charisma.
Less damage on the bite/grapple: 15 (2d10+4)
The regular immunities of a werecreature, along with the Hold Breath
think that would make an appropriate challenge for a level 4 party?

[ravencroft0]

How many hit dice does your werecroc have? If you intend to use the giant croc's listed stats, they are rated for a 9 HD creature. If your were is going to be a low level base creature to start with, then sure the 18 Strength is fine. If you keep its stats for the listed giant croc, then you need to keep the Strength at 21. The giant croc is a CR 5 creature. Giving that croc human level intelligence could easily give make it CR6. The bite damage is throwing me, too. I don't feel you should drop it to 2d10. The werebear has a 2d10 that's based on the base creature. I understand you want to keep the whole thing balanced for a low level party, but I really don't think your group will be able to take down this guy and potentially every other beastie and enemy NPC in the lab. You pretty much are looking at the Killer Croc from DC comics, which sounds like an amazing encounter, but very dangerous. I would hate that you would have to water down your encounter just to prevent a TPK. Hope it goes okay for you!