I havent ordered a felt so I don't know how they usually do things. A bike could be unavailable for a number of reasons. They could have made too few, or the bike in your size was more popular than expected. I'm guessing most companies get frames made in batches throughout the year. If one sells out you may have to wait until the next production run.

I ordered a Trek Crockett frameset in January. The distributor had 1 or 2 rim brake frames in stock for immediate shipment. To get a disk brake frame I would have had to wait until April or May. If I wanted a disc frame I would have had to wait 3-4 months.

If the F4 is the bike you really want you have to wait until more become available or try to find a different shop that has what you want now.