It's an i3 ....... what 4 cores are you talking about?
He was probably assuming i3 is quad core, since Core 2 ended up with 4-cores and there's no logical reason to assume i3 would be less cores. You would have to know that i3 is simply re-branded Core 2 Duo in order to know that i3 is only two cores, and that's not really ultra-common knowledge. In fact, it's pretty stupid on Intel's part.
However the situation doesn't change based on his error - make sure all (or, for technicality sake, BOTH) of your cores are enabled in the BIOS