What's a 4 ohm setup?

Infact what is an ohm?

I've thought about this instead...

The same HU:

http://iweb.alpine-usa.com/pls/admn/...y=45&p_main=10

The same Type-R speakers:

http://iweb.alpine-usa.com/pls/admn/...y=90&p_main=10

And this amp:

http://iweb.alpine-usa.com/pls/admn/...=120&p_main=10

Just that setup, would that be the perfect setup, not under powered, not over powered, just right, to let me at a later date pickup a dedicated amp for the sub, or maybe buy an even better sub? It's just personally, I don't want that much weight in my car, since it's already pretty damn heavy and slow.

Or would really, the HU + Type-R's + Type-R Sub and the Hifonics amp really be good? Not over or under powered. I've gotten mixed answers difference places that the Hifonics amp won't be enough for it all, and to just get 2 different amps, one for the speakers, and one of the sub.

And it's a 1998 Nissan Altima.