These are the new stats for Arktos for this level. I haven't done anything so far about your level 4 upgrade in regards to Beast Rider yet. I'm still looking into that.
On top of what's below, Arktos gains a +1 to one of his ability scores.
If it's his Dex, add +1 to AC and +1 to Reflex saves
If it's his Con, add +4 to his HP and +1 to Fort saves
He also gains one additional skill point.
He also appears to only have one trick, when his level indicates he should have two bonus tricks.
[ +- ] Arktos
"ARKTOS"
Ghost Bear Mount
HP: 22/22
BAB: +3
Size: Large;
Speed: 40 ft.;
AC: 16 (+4 natural armor);
Attacks: bite +6 (1d6+3), 2 claws +6 (1d4+3);
Ability Scores: Str 16, Dex 15, Con 13, Int 2, Wis 12, Cha 6;
Special Qualities low-light vision, scent.
Fort: +5 Reflex: +6 Will: +2
Skills: Acrobatics (+1), Perception (+1), Intimidate (+7)
Endurance [Feat]:
You gain a +4 bonus on the following checks and saves: Swim checks made to resist nonlethal damage from exhaustion; Constitution checks made to continue running; Constitution checks made to avoid nonlethal damage from a forced march; Constitution checks made to hold your breath; Constitution checks made to avoid nonlethal damage from starvation or thirst; Fortitude saves made to avoid nonlethal damage from hot or cold environments; and Fortitude saves made to resist damage from suffocation. You may sleep in light or medium armor without becoming fatigued.
Valiant Steed [Animal Feat]:
Your courage is unnaturally strong.
Prerequisites: Animal or magical beast.
Benefit: You gain a +4 morale bonus on saves against fear and emotion effects. If you can serve as a mount, your rider gains a +4 morale bonus on Handle Animal, Ride, and wild empathy checks to force you within range of a creature with the unnatural aura universal monster rule. In addition, the DC to push you with the Handle Animal skill does not increase if you are wounded or have taken any nonlethal damage or ability score damage.
Normal: The DC to push an animal increases by 2 if the animal is wounded or has taken any nonlethal damage or ability score damage.
Power Attack [Animal Feat]
Choose to take a –1 penalty on all melee attack rolls and combat maneuver checks to gain a +2 bonus on all melee damage rolls. When your base attack bonus reaches +4, and every 4 points thereafter, the penalty increases by –1 & the bonus to damage increases by +2.
Attack [Animal Companion Trick (DC 20)]: The animal attacks apparent enemies. You may point to a particular creature that you wish the animal to attack, and it will comply if able. Normally, an animal will attack only humanoids, monstrous humanoids, giants, or other animals. Teaching an animal to attack all creatures (including such unnatural creatures as undead and aberrations) counts as two tricks.
Link:
A beast rider can handle his animal companion as a free action, or push it as a move action, even if he doesn?t have any ranks in the Handle Animal skill. The beast rider gains a +4 circumstance bonus on all wild empathy checks and Handle Animal checks made regarding an animal companion.
CHARGER ARCHETYPE:
http://www.d20pfsrd.com/classes/core-classes/druid/animal-companions/animal-companion-archetypes/charger-companion-archetype/
Chargers treat Intimidate as a class skill.
Mounted Challenge (Ex): When ridden by a cavalier, chargers gain half of the bonuses and penalties granted by the cavalier's challenge class feature.
Barding Training (Ex)
At 3rd level, a charger’s armor check penalty for wearing barding or for carrying a medium load is reduced by 1 and the maximum Dexterity bonus for its armor and load increases by 1. Reduce the armor check and medium load penalties by an additional 1 for every 6 levels after 3rd; increase the maximum Dexterity bonus for the charger’s armor by an additional 1 for every 6 levels after 3rd. In addition, the charger’s speed does not decrease when wearing medium armor or carrying a medium load.