Laptop Model??
Correct my if I'm wrong - You have 2 Gig Ram in slot 0 and you added 4 gig ram module to Slot 1.
(1) What Ram did you buy, ie DDr3-1333 CL9 ???
.... May have a problem if speed/timing is different between the two modules.
(2) Did you verify that the Laptop supports 4 gig modules.
4 gig limitation, has already been addressed. For total of 4 gigs you can use either 32 bit or 64 bit OS (64 Bit will allow 0.5 more ram to be available - NOT a Biggy. To See/use more than 4 gigs (ie 6 Gigs) need 64 Bit.
Recommend you remove the 2 gig ram Module and put your 4 gig Ram module in place of. This should allow you to boot and see the 4 gigs, If this does not allow you to boot then the Ram is either not compatable or defective.
... In any case, to run in daule channel mode both STICKS must be the same size and ratings need to be identical. The prefered method to upgrade from a single module is to Buy a 2 module kit and replace the single module.
Bottom line - (1) Return 4 gig Module and get a 2 gig module that is IDENTICAL to your orginal 2 gig module. (2) 2nd option, if single 4 gig module worked, use computer that way.