By design, the 4x6 DD coil is optimized for mineralized ground with its narrow sense zone and gives up some depth in non-mineralized ground when compared to a concentric design. On top of that, the 4x6 is a bit smaller coil (equivalent to a 5" concentric) which will also affect depth.
You don't say anything about your disc. settings, gain etc....all of which could also be affecting your success. Which minelab was your friend using? I once had an Etrac that was a silver hog, but in trashy areas wasn't a good performer (for me)
My point being there could be things besides the coil that come into play that contributed to your perceived competitive disadvantage.
As for myself, with my MXTPRO and 4x6 DD coil, I think getting 5" depth would be a pretty solid performance considering the mineralization in my area, so to me it seems like you are getting pretty solid depth performance.
My favorite trashy coil is the 8x6 SEF which I use unless the trash/unmasking problems just overwhelm me, then I go for the 4x6DD. The advantage of the DD design is such that it can let you get away with a bit larger coil (in the case of the SEF it is equivalent to a 7" concentric) in trash as it has such a narrow detection zone which would give more depth than a 6" concentric. the SEF coil is no larger in one axis than the 6"concentric but longer by 2" in the other so by rotating around the target you should be able to find an angle that allows you to get as good a look as the concentric, but gives you better depth.... .....theoretically that is.....
As with all things detecting, its always a balancing act for maximum performance on any given day, in any give area. Sometimes you will have the big gun, sometimes a plinker....this is how detectorists get sucked into owning a stable of detectors and coils. IMHO
Good Luck
