I don't use the 4" but I have the 5.75" concentric coil mounted on my Vaquero most of the time and I love it.
In trashy sites it works better than just well, in iron infested sites, one crazy phenomenally iron filled site in particular, it took a little practice and slow coil movements but eventually I learned to pick out high conductors in a sea of very dense iron fairly easily with great success.
The fast response and separation abilities on unmasking targets is great on all Tesoros...it seemed magnified to a noticeable degree using this small coil so I assume using that even smaller one it will be just as good and enjoyable...if not better.
Surprising depth too and I have used mine in both almost perfect and fantastic soil in the midwest plus the red devil dirt in the southeast.
Mine seemed pretty lucky for me also, here is a movie I made after finding a very small gold ring in a very large area that no way I had any business even trying.
https://m.youtube.com/#/watch?v=FhPT55m2TOM
This was only my second or third hunt using this coil and I had hunted this area before with larger coils and other detectors so it was pretty lucky, I love using my skills to find great targets but when luck comes my way I embrace it fully.
I was moving pretty fast but I still got a great, solid, loud dig me signal on this tiny thing with my disc set a little lower almost to iron so I went back to examine it and using my thumbing technique to pass the fade out point on the disc knob and back down it came in super solid with no noise so I had to dig it.
The sharp disc abilities on this small coil is as good if not even more sharp using this coil as any I have ever used including the great disc on my Compadre.
I prefer the concentric over the widescans because I hunt many trashy sites with lots of pop tops and this kind works well in that environment.
I hunt mostly trash filled or iron filled sites, many with both, so the smaller coverage does not bother me at all, it actually fills me with confidence instead.
A valuable tool in my arsenal, I hope you will discover yours will be the same.