My group currently has 3 players, and I've debated this exact dilemma. The solution I'm going to take involves every option you listed (except #1 which I feel would tend to dilute roleplaying and promote metagaming).
1. Bulk up the PCs slightly. Give 'em an extra magic item, healing potions, etc. If that's too much of a stretch for your campaign, at least make sure the PCs have access to some kind of reliable healing back at their base of operations.
2. Weaken the opposition slightly. Have them make mistakes, and roleplay it. It really lends an air of realism if you have the enemy make an occasional mundane mistake like forgetting to reset a trap, thinking the PCs are someone else, or whatever. Apply random damage to the bad guys; they have their own adventures, too, and can't always be in tip-top shape.
3. Offer campaign-friendly ways to incorporate nameless henchmen, i.e., the town wainwright thanks the PCs for agreeing to solve the riddle of the haunted tower, and he sends his two strapping sons (both Ftr1) with them. Then play the NPCs as brave enough to stand their ground but not so foolhardy to become trap-springers for the PCs.
4. Last and best, play to the obvious strength of a smaller party: deeper roleplaying and more individual subplots. Big groups can get bored with lots of roleplay because it usually involves only a few of the players at any one time. Likewise, individual subplots can get marginalised easily in a large group; but in a very small group, each player can hog more of the RP spotlight without ruining the game.
While I think any style of adventure can be run for a small group, I imagine you'd probably have the most success with roleplay-heavy ones set in well populated areas, rather than combat-centred adventures in remote, hostile lands.
ironregime