The 4" concentric coil is fairly new and was released without any fanfare at all. I like it on my EuroTek Pro and on my Omega 8500. Its a good trashy hot spot coil that maintains its small foot print.
The 5" DD coil is a good coil too. I like that it gets a bit more ground coverage than the 4" concentric. While the 4" coil is more like a bullseye, like hunting with a 50 cent piece, the 5" DD has a wider field and so you get a little more ground coverage. The trash dictates what you use as these are both really high trash coils.
When I sold my F44, I sold it with the 9" coil and kept the 11" Triangulated coil it came with. I also use it on my Eurotek Pro and my 8500, and I also use it on my F75 when i want a bigger concentric coil on it and am ok with an on/off audio response (no modulation).
All that said....The 4" concentric is a decent coil and its cheap as coils go if you want a budget minded trash coil. But you won't go wrong with either one.
HH
Mike