BOARZ IZ BEST!
When adding monsters together, think less about the CR and more about the experience the monsters bring upon defeat. A Worg w/the Simple: Giant template is a CR 3 monster, meaning it delivers 800 experience upon it's defeat. If you add in an orc worg rider (a CR 1 monster) this adds another 400 experience to the total for the encounter.
800 + 400 = 1200 experience total for the encounter
This means the encounter value of these 2 monsters together is a CR 4 since the full encounter delivers 1200 experience, the same as a CR 4 monster.
If you want more monsters on the table and are willing to use the Simple: Giant template, consider the following:
Goblin Fighter 1 + Brutal Goat (goat with Simple: Giant template applied)
You have out-of-the-ordinary combatants, you might add some interesting terrain to give the goats a slight advantage and each individual in the group adds 200 experience.
A sample encounter might be:
x2 goblin fighter 1
x2 brutal goat
x2 goblin warrior 1 (armed with shortbows)
x1 goblin adept 2
The total experience for this encounter would be 1270; slightly higher than CR 4 but not enough for CR 5. However you could round down to CR 4 and call it that.
Essentially the Adept hides and has his familiar delivering Guidance or Touch of Fatigue spells; perhaps he hurls a single Cause Fear. The 2 goat-riders ignore 5' of difficult terrain and set themselves into some charging lanes with small boulders - they can charge but their enemies cannot. Finally the warriors focus their attacks on ranged attackers and spellcasters.
You can put together all of this for the same cost as a single Large worg and Orc worg rider.