Hi,
those are some really nice detectors you have picked out. Teknetics website has the Patriot(=F70) and the T2 Classic at prices that are well within your price range. They are a big step up from your F4 and those are new detectors with a 5 year warranty. It would be hard to find a new AT Pro under $500 but used would definitely fall in your range. For $50 more than your upper limit you could buy a brand new Equinox 600 from several online dealers. Since the Equinox models were introduced the prices for the X-Terra 705 have dropped quite a bit. You could easily find a new one or nearly new to fit inside your price range and maybe get more than one frequency coil.
The Patriot and T2 Classic are great detectors. They are quite a bit more sparky than your F4, can get a little noisy in bad EMI and can be hard to tame in the beginning. Once you are used to them they are very capable coin machines and are excellent for relics too.
The AT Pro is also a great detector. If you don't need waterproofing though why would you need one? Part of the price you pay for it involves the waterproof features. If you are undecided about waterproofing you can get a new Equinox 600 for a similar price which has a lot more features than the AT Pro. I have used both and still own the Equinox. It has not made any comparable detectors obsolete, but I can tell you for a fact it sure makes it hard for me to want to hunt with any of my other detectors including my XP Deus.
The X-Terra 705 is an amazing detector because of its ability to change frequencies by using different coils. My only beef with it is the recovery speed/target separation felt sluggish to me compared to the Teknetics and especially the Nox 600. Otherwise, it is a versatile, deep, very refined detector that can do just about anything except submerged water hunt.
You really can't go wrong with any of these. If you decide it's just too much money at the moment the F44 is a smaller but good step up from the F4 too and $150 cheaper than the T2 Classic. Personally I would buy the Patriot (F70)
since it is probably the best detector on the market for less than $400 new.
Jeff