No, I could not find a bonus to multiple limbs in the rules.
If you're looking for a house rule, you could say you get a +2 to CMB for every additional limb beyond 2 (like animals get to CMD for being on all 4's).
However, if this is an eidolon or a creature you're defining, adding the Grab ability and MultiAttack monster feat would be hugely beneficial:
Grab (Ex) If a creature with this special attack hits with the
indicated attack (usually a claw or bite attack), it deals normal
damage and attempts to start a grapple as a free action without
provoking an attack of opportunity. Unless otherwise noted, grab can
only be used against targets of a size equal to or smaller than the
creature with this ability. If the creature can use grab on creatures
of other sizes, it is noted in the creature's Special Attacks line.
The creature has the option to conduct the grapple normally, or simply
use the part of its body it used in the grab to hold the opponent. If
it chooses to do the latter, it takes a –20 penalty on its CMB check
to make and maintain the grapple, but does not gain the grappled
condition itself. A successful hold does not deal any extra damage
unless the creature also has the constrict special attack. If the
creature does not constrict, each successful grapple check it makes
during successive rounds automatically deals the damage indicated for
the attack that established the hold. Otherwise, it deals constriction
damage as well (the amount is given in the creature's descriptive
Creatures with the grab special attack receive a +4 bonus on combat
maneuver checks made to start and maintain a grapple.
Format: grab; Location: individual attacks.