CR 4 bear, +2 CR from Greenbound would make him CR 6.
Let's look at the finished Greenbound Brown Bear
MM-Forgotten Realms said:
Bear, Brown
Large Animal
Hit Dice: 6d8+36 (63 hp)
Initiative: +3
Speed: 40 ft. (8 squares)
Armor Class: 23 (–1 size, +2 Dex, +12 natural), touch 11,
flat-footed 20
Base Attack/Grapple: +4/+23
Attack: Claw +14 melee (1d8+11)
Full Attack: 2 claws +14 melee (1d8+11) and bite +9 melee (2d6+5)
Space/Reach: 10 ft./5 ft.
Special Attacks: Improved grab
Special Qualities: Low-light vision, scent, tremorsense, DR 10/Magic and Slashing, Fast Heal 3, Resist 10 Cold (ex), Resist Electricity 10 (ex)
Saves: Fort +11, Ref +7, Will +3
Abilities: Str 33, Dex 15, Con 23, Int 2, Wis 12, Cha 10
Skills: Listen +4, Spot +7, Swim +12, Hide +16 (in forest or wilderness)
Feats: Endurance, Run, Track
Environment: Cold forests
Organization: Solitary or pair
Challenge Rating: 6
Advancement: 7–10 HD (Large)
Level Adjustment: --
Let's compare this to a 6th level Barbarian (another 6 hd brawler)
My basic rule for PC armor class is level +14 as the minimum to stand on the field. This guy is 2 points over the minimum, which means in the low-to-medium range for a front line fighter.
Hit point wise: Treating his hit dice like D12s, he should have 12 + 5x 6.5, or 44.5 plus reasonable CON. He's 18 points over that, which would be right for a 16 con. That sounds like "reasonable" to me.
Base move of 40. That sounds like a Barb.
Attack at +14 sounds like a 6th level Barb, in his dreams, with a secondary attack at +9. The extra main attack at +14 is a bonus our theoretical Barb doesn't have.
DR 10/Magic+Slash is a bonus ou theoretical Barbarian doesn't come with.
Energy Resistance 10 v Electrical is a bonus our Barb *could* have, presuming a low level ring appropriate to a level 6 character. The 10 point v Cold is a bonus harder to match.
Improved Grab is a Feat the Barbarian could have, though I think he needs Improved Unarmed Strike for the Barb to have it. Said Barb probably isn't in the +23 weight class though, unless he were Enlarged. Then he's about +21 or +22. Rage and he's in there.
Damage of D6 + 11 is sort of like D8 + 10. The Barbarian would have to Power Attack to get that damage with a long sword or axe, and if he went with a 2-handed weapon his AC would fall. Advantage to the Bear on this one.
Feats of Endurance, Run and Track for the Bear, on top of the Improved Grab and the implied Improved Unarmed Strike is way more feats than the Barbarian could have. To be fair, the Barbarian will probably have more skill points over all, so there's something of a balance there. (Okay, I lied, most of those skills won't help in combat, which is what we're looking at.) Advantage Bear
No real way to match Tremorsense for our Barbarian. It effectively negates a lot of the advantage that things like a good Hide check or Invisibility might give. There isn't a feat, skill or magic item that matches. Advantage Bear
+16 to Hide in wooded areas can be huge, but probably wouldn't come into play in the case of a Summoned creature. Call that one a wash.
Comparing Saves:
The Barb gets +5 Fort from levels, +3 from Con, for +8 total. Bear has +11. Advantage Bear
The Barb gets +2 Ref from levels, probably +2 from Dex, for +4 total. Bear has +7. Advantage, Bear
The Barb gets +2 Will from levels, maybe +1 from Wis, for +3 total. Bear has +3. Tossup.
In a straight brawl, no magic spells, no meaningful use of things like alchemical fire, cold or acid, the bear wins by virtue of damage and grapple. Kick in rage and Barbie's damage goes up, but so does the damage the Bear can deliver because of the AC loss, and since the bear has an extra attack at full bonus, the Barbarian's Rage actually favors the bear slightly.
Rarbie is going to hit about 50% of the time (calling him +11 to ht). Bear has a +14 to hit, su call that 2/3 of the time.
Round 1, Barb does D8+5 x 0.5 (50% hit rate) + D8+5 x 0.25 (25% hit rate for secondary attack. Average 7 points per round +-. At that rate the bear falls in 9 rounds.
Bear does D6+11 x 0.66 (better to-hit v similar AC) + D6+11 x 0.66 + 2D6+5 x 0.4, or about 20 points per round. At that rate the Barb falls part way through the 4th round.
Things like Cure Light potions won't help. Gain a D8+ points while taking 20 average at the same time. Cure Serious is a little light of a break even. I think we can discount any self-administered healing the Barb can do.
If the Barb Rages, his HP rise by 12, and his attack and damage each go up by 2. He's now hitting at close to the bear's level (+13 to hit v +14 for the bear). His damage per round goes up accordingly. Not enough to match the bear's, but some.
The bear, on the other hand, also starts hitting more often, about 3/4 of the time, which effectively raises his per-round damage. Because of his extra full-bonus attack, he comes out a winner on this exchange.
And we're not even considering the bear's Grapple attack, which is automatic on any claw hit. If he wins that grapple check he gets to do both claw attacks of damage automatically, for about 29 points per round until the Barb breaks the hold. Simply stated, after the initial hit, the Barb dies in about two rounds of that. Bear has about a seven point advantage in that area. Not insurmountable, but solid.
Bottom line, presuming that the Barb faces this Bear alone, has an AC in the low 20s (comparable to the bear), and has at least a +1 Sword or Axe to bypass the DR, my read of this is that the Barbarian is not eating bear steaks for dinner that night. Rather, the bear and her cubs are dining in style.
So my read is that the package, in this case, is worth more than a +2.
Give the Barb another level, meaning probably +9 HP, another point in AC, another point of BAB and maybe another plus on his weapon and the story changes. He lasts four solid rounds, presuming no Grapple, and the bear falls in 8. Same outcome, it just takes longer.
That's my run of the numbers. If I missed something, please correct me.
<Edit> I did miss something: I forgot the Fast Heal. That effectively drops the Barb's damage by 3 per round. This fight is even more one sided than i thought. </Edit>