I'd get a more powerful single GPU over Crossfire if you can.
O.k, so let's answer a few of your questions, to get them out of the way. No, you can't Crossfire (2x) R7 250X's with (2x) R9 280x's for a Quad-CrossfireX configuration. CrossfireX can only work within the same GPU family architecture (Ex: 4x Tahiti, 4x Hawaii, 4X Fury etc.). So your options would be either in a dual CrossfireX configuration, not in a Quad-CrossfireX configuration.
Example #1: Two R9 280x (Tahiti) would Quad-Crossfire perfectly regardless of the manufacturer, with two R9 280 GPU's (Tahiti), but would utilize the available video memory space of the lowest tiered card if they differ.
Example #2: Yet, Dual R9 280x (Tahiti) and two R9 290x (Hawaii) would not, even though they are both GCN architecture based, because they are from differing GPU family's.
So in your example listed within your original post, this setup wouldn't be capable of a quad-fire configuration, because they are incompatible solutions.
Now onto what warpfact0r10 began to touch on. I agree that if you haven't already built this system, you should definitely go with a more powerful single card configuration such as an R9 290(X) 4GB solution or higher. Especially so, when games are demanding more video memory space, so you would definitely benefit from having 4GB or more video ram available as well as a more powerful GPU core, it would be your option for the best investment overall that you could make to future proof your system. Considering the price of two R9 280x's would be roughly the equivalent of a single R9 390x, I'd have to say that would be your best option as it contains 8GB of video memory which would serve you quite well, both now and in the future.
If you'd like us to assist you in your build, we'd be happy to help. Please leave an estimated budget and we'll begin to compile a build that would both suit your needs, as well as be future proofed to save you headaches in the future.
Anyways, I hope this helps and have a nice day.