The 4 is the number of bolt holes, the 100 is the spacing between them. Only 4x100 will fit but also watch out for the hub centerbore - the hole in the middle of the wheel. It must be 57.1 mm or larger. If it's larger you must then use a spacer ring called hubcentric rings to make the hub centerbore smaller to 57.1 mm. Most aftermarket wheels need a hub ring to fit because they manufacture them with larger hub bores so that they can fit many different cars.
In short, you can only use 4x100 wheels unless you have an adapter plate or switch the hub, which I do not recommend for just wheel compatibility.
You can but I don't recommend it. An adapter generally moves the wheel hub outward, changing the offset of the wheel. In other words, the wheel gets pushed out. Even if you get a wheel that fits, this changes the suspension geometry. The brake rotor won't work either, you have to drill holes in it.
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