Well, I'm not sure about the price-range in the U.S., but here you can't get a single-speed for under 400€ (counting with €~$). Trek has the 7100 FXS hybrid for about 500€ (probably $500 in the U.S.), but you could get a Trek 3700 that seems to be a fair compromise between price and quality for 400€, the 4300 is of course a step up the ladder, but also more expensive. It would seem, that in general hybrids are more expensive than MTB:s.
From your description of usage, I'd recommend a hybrid, but I've always thought that MTB:s are just as versetile and that most hybrids look like granny-bikes, even though I currently ride one (fitness-hybrid).
The 4300 does have front-suspension going for it, so in your shoes, I would go for that one.