[*]3-4" blade
[*]reversible clip
[*]lock back preferred
[*]one handed opening
[*]<$100
As I said, I'm a newb so be kind.
I fear I'm searching for the Holy Grail...
Don't worry, there's plenty of choices out there with everything you're looking for. As far as lockbacks that fit the bill, Spyderco has a lot of options:
Delica/Endura (Endura's just a little bigger)- Can't go wrong here. Won't break the bank, has all the feature you listed, and there's a lot of variations available.
Native- Not as many variation's as the Delica & Endura, but still plenty. Wal-Mart used to (and may still) carry an FRN handled version with S30V steel for I think around $50. Worth checking there so you could handle one; see if it agrees with you.
Stretch- More expensive than the others, but you should be able to find one well within your budget. Haven't handled one personally (I don't usually gravitate toward lock-backs), but from looking at pictures/specs, I really can't find anything I dislike about it. Seems like an excellent knife.
Caly 3/3.5- A little above your budget, but definitely worth considering. Great knives.
I'd also reccomend just browsing Spyderco's website, and see what jumps out at you. For good modern lock-backs I don't think any other company comes close to the number of models they have.
On crimsonfalcon07's recommendation of the Paramilitary 2, it's an excellent knife (There's one in my pocket now). It might not be quite as lefty friendly as a lock-back though. It's not hard for me to close left handed, but it is just a little awkward. That said, I'm right handed so most knives are a little awkward to close left handed for me. I'd check one out if you can, see how it feels for you. Or maybe a lefty could chime in with a little more insight.
Sorry if this seemed like a big Spyderco ad. Like I said, I don't usually gravitate toward lock-backs, and Spyderco's are just the one's I'm most familiar with.
If the preference for a lock-back was mainly ease of use left handed, there's a lot of other lefty friendly locks out there aswell: axis lock, ball-bearing lock, left handed liner and framelocks etc. Not trying to push any of them over a back-lock, I know a lot of people prefer them, but it's something to consider.
Have fun on the search, and welcome to the forums.
Edit: Oops, sorry for the repeat suggestions. Got sidetracked for a while in the middle of my reply and didn't check for new posts.