Need help with challenge rating

[Tinq]

I developed a monster for my current group, but I'm not sure what CR to give it. It's meant to be faced in groups of 3-4, so I want each individual monster to be under level 6, where my group is, but I'm hoping this beast hits around CR 4. Tell me what you think.

Grizock

Size/Type:
Small Magical Beast

Hit Dice:
4d10+11 (33 hp)

Initiative:
+4

Speed:
30 ft.

Armor Class:
19 (+4 DEX, +4 Natural, +1 Size)

Base Attack/Grapple:
+4/0

Attack:
Bite +9 melee (1d8 plus poison)

Full Attack:
Bite +9 melee (1d8 plus poison)

Space/Reach:
5 ft./5 ft.

Special Attacks:
Improved grab, Paralyzing bite

Special Qualities:
Blindsight

Saves:
Fort +6 Ref +5 Will +2

Abilities:
STR 10 DEX 18 CON 14 INT 2 WIS 13 CHA 10

Skills:
Hide +15, Move Silently +11

Feats:
Weapon Finesse, Toughness

Environment:
Underground

Organization:
Pack (3-4)

Challenge Rating:

Treasure:
None

Alignment:
Neutral

Advancement:
-

Level Adjustment:
-

This eyeless creature resembles nothing more than a large, grinning fanged mouth on four legs. What parts aren’t teeth are covered in a hardened shell.

The average Grizock is 3 feet long and 1 to 1 – ½ feet high. They are actually white, but covered so completely in dirt that they blend in with their surroundings. Though blind, they navigate by echolocation, emitting an inaudible whine at all times.

Combat
Grizocks are not savvy combatants. They attack any prey that wanders near them, latching on with their massive mouths and grinding their teeth. They always use their Paralyzing Bite Extraordinary ability at their first chance.

Improved grab (Ex):
To use this ability, a Grizock must hit with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, the crocodile establishes a hold on the opponent with its mouth. While grappling, the Grizock is able to do its bite damage to its opponent once per round.

Paralyzing bite (Ex):
A Grizock makes paralyzing venom and, once per day, it can inject that venom into an open wound when it bites a victim. This is a free action that happens with a successful attack. The venom causes its victim to become paralyzed. A DC 16 fortitude save negates, but the victim remains sickened for an hour. The paralyzing effect wears off after 10 minutes, but the victim becomes sickened for an hour afterward.

 

[udalrich]

You forgot to replace at least one reference to crocodile with Grizock. But lets compare with the CR 4 Giant Crocodile.

HP: 33/59. Giant croc is twice as good.
AC: 19/16. Grizock is better.
Speed: 30/20 (swim 30). About even, depending on the environment
Grapple: +0/+21 Giant croc is much better.
Full attack: +9 (1d8+poison)/ +11 (2d8+12) or +11 (1d12+12). Giant croc is much better.
Reach: 5 ft/10 ft: Giant croc is better
Specials: Improved grab, blindsight, paralyzing bite/improved grab, low light vision, hold breath. Blindsight is much better than low light and the poison is better than hold breath.
Saves: 6-5-2/9-6-3. Giant croc is better.
Skills: Hide +15, Move Silently +11/ Hide +1-+11, Listen +5, Spot +5, Swim +16. Grizock is better.

In most cases, the giant croc is better or hugely better.

As written, improved grab is almost useless. It can only be used against creatures that are Tiny or smaller, since the grizock is Small. With the +0 modifier, it is almost certain to fail to grapple anybody who is on the front line. And based on the description of its tactics, it probably is not going to get to the squishy wizard hiding in the back.

The only thing that might bring them up is the poison, which is potentially very strong. Front-line fighters will probably save against the poison, and sickened is not a huge penalty. However, a few unlucky rolls could leave the rogue and the wizard fighting without other help. Are they smart enough to Coup d'Grace a foe that fails the save against their poison?

To be CR 4, I would add 1-2 hit dice. This will help the hit points, saves and grapple modifier. I would add a racial bonus of probably about +4 and give it a bonus feat that allows it to use its dexterity modifier for grappling. That puts its grapple modifier at about +9, which is probably about a 50-50 chance to grapple a 6th level fighter (+6 BAB, +4 strength). The damage is still low, but if one or two of the PC's is unconscious, that will balance it out.