I assume you mean CR?
There's really no easy way to answer this, beacuse a)You don't mention what stats this character will have, making it hard to judge power level, and b)CR is a very rough quideline. CR/4 might be ok at level 1 or 2, but things would quickly get too easy... "Appropriate" CR - 4 sounds about right, but keep in mind that a critter using poison or a grappler going after a mage, or an undead monster facing a rogue will be a lot tougher for them than CR will indicate - they're meant to be fought by a whole party.
Still, here are a couple of the things you should keep in mind if you intend for this character to survive:
1. Look for monsters and enemies that can put up a fight (decent AC, decent HP) but don't dish out a lot of damage.
2. Be very careful when using magic against the character - a single "Save or Die" spell (Sleep, Hold Person, or anything else that stuns or disables for several rounds, in this case) can end the character's career instantly, because there won't be anyone around to cast Dispel Magic at him or protect him until he comes to. So unless you want him to get "resurrected by mysterious stranger" or "captured by enemies only to escape again" you might want to be careful with that.
3. Provide him with some sort of renewable healing item (family heirloom, gift from his master, whatever) - he'll need it.