PCB manufactoring-There shouldn't be bottom silkscreen selected?
This likely means that when you specified the board at the 4pcb site, you probably told them "top silkscreen only", or are using a purchasing option that does not allow for a bottom silk screen.  This is usually the default option, and the ONLY option if you are using some low-budget option.  I haven't used the 4pcb service in some time, as I like some others better, but IIRC, the $33 boards could not have a bottom silk screen. 

A bottom silk screen (which means you want printing on the bottom layer) is an extra manufacturing step.  Most, if not all, board houses will do it, but you will pay extra for it.  Because it is more expensive, its often not allowed in the rock bottom pricing options, which are automated to the largest extent possible.