The 'offset' is the distance between the mounting face of the wheel (i.e. the bit that sits against the hub) and the centreline of the wheel. A positive offset is where the mounting face is towards the outside of the wheel, and negative is set towards the inside.
Therefore, if you get wheels that are more positive than the current ones, they will sit further in on the car (and may rub on the suspension), and ones that are more negative will sit further out (and may rub on or stick out of the arches).
The other thing you need to be aware of is the centre bore - the size of the hole in the middle. Many cars are designed so that the wheel sits over a slight stub on the face of the hub, which takes some of the load off of the wheel bolts. If your new wheels have too large a bore, that won't happen and you risk damaging the bolts - obviously if the wheels have too small a bore they simply won't fit!
4x100 is a common pattern here in Europe, particularly 90s hatchbacks, but I don't know about the states. I suspect there are websites around listing common patterns - there is certainly at least one for UK-spec cars.
