This will risk the chance of adding smaller flashers and fairies in the future. Most flashers are peaceful however the Octotaenia can be slightly more boisterous and depending on tank size may not allow other smaller similar species (Flashers or fairies). The 4’ tank will be a small issue as I have had a 5.5 inch fairy in mine and he was extremely active in it, IME, flashers are twice as active as a fairy is and so this specific flasher will ideally want a 5’ if not 6’ tank.
If you add fairies first, then you will want to choose wisely and monitor behaviour to see if you will be able to add flashers layer down the road. Those Cirrhilabrus you have mentioned should be alright but I would wait for the LFS to get in a blue flasher again and reserve it ASAP then once you add him, go for another flasher if you want more - in a 4’ tank I’d say 3 or 4 flashers would be a maximum and 4 or 5 fairies depending on other stocking and rockscape.