1a) There's only one character, so incapacitation = death.
1b) Because of the above, status effects like Stun/Dominate/Daze/Grab are really going to hurt him, due to the economy of actions. Use these things very sparingly.
2) How successful he is in combat is going to depend on what his role is. The Leader role, for instance, really isn't all that great on its own; the leader's powers revolve around augmenting allies. With no allies, he's a cart without a horse (this can be overcome by giving the leader some NPC minion allies, or figure(s) of wondrous power). A controller is going to have the hardest time because he has little staying power. Comparitively, a defender has a lot of staying power (but his Mark is relatively useless). A striker will have the staying power, but without allies, continually gaining combat advantage will be difficult for a rogue. The others, not as badly.
Thus, certain monsters are going to be real problems, depending on what they are. Defenders do well against brutes and soldiers (the damage and defenses are decent); Lurkers and Strikers have few HP, but the amount of damage they do goes well against the Defender's HP (assuming they can do their extra damage thing). Defenders have no ranged abilities, so monsters at range, in a defensible position, are going to be a serious threat. Strikers are going to fold under a brute, but might do well against a skirmisher, controller, or artillery. Controller PCs can do well against artillery and controller monsters; it's like a shootout between two gunslingers.
3) Despite #2, be aware that you can structure an adventure to suit the class. The adventure could be a lot of skill usage, roleplaying (The Warlord commanding an army, the Cleric doing things for his church like interrogating witches, building churches, spreading the word far and wide). The wizard can conduct investigations, research, etc.
4) IMHO, 4 minions are not a threat to the defender. He has a bunch of HP, and assuming he hits every round, can chew through them before he's in trouble. Strikers suck against minions (they can't do their extra damage), and with their limited HP, it becomes more of a challenge. Controllers, it depends on the setup of the minions; if they can get an area attack off, the minions are gone without a threat.
Minions with ranged attacks are very, very dangerous.
My advice: If you're using a defender: 4 minions + standard monster (artillery, skirmisher or soldier). Anyone else: 1 standard monster, or 2 minions + standard monster 2 levels below the PC.
WHEN IN DOUBT: Playtest the encounter before hand, using hte character's sheet vs. your monsters, to avoid
5) Monsters with vulnerabilities/resistances: avoid. If it has a vulnerability your player has, then it's going to die, quick. If it has a resistance against something your player regularly uses, it's going to take forever to kill.
6) Terrain that the PC can use to their advantage. Cover, difficult terrain, and hazardous/obscuring terrain are very useful. Assuming the PC is clever.