What is the F/B RX ratio of the 4SQ you use?
Thanks for the question at talk. Never been asked that one before.
It is all a bit theoretical but F/B ratio > 20-25dB should be dead easy to obtain and 30dB is not impossible. Some have claimed > 45dB if you get the phasing and separation right but unless lucky I think that is pushing it a bit and when DX you don't quite have that long to tinker and have to make do with something close.
When as i mentioned in the talk when DX I can tell you you absolutely CAN null out one direction or another and it is possible to have a qso with a station in one direction (qrp) whilst in another there is another station at 59+
I am pretty sure there is something more recent but NCJ covered options a while back including add a central 5th element (I never got it to work and 4el were enough for me)
A 4 sq should get you SOME gain in one or more direction on TX but it is the RX F/B gain you want.
An extra note:
There is SOME TX gain but I never advise on any existing (think of it like a 1/4 wave vertical). I don't know if I even said that in the talk.. The RX gain F/B is what blew me away first time I heard it (6W/M1KTA station on 40m I could qso with JA clear as a bell (when EU played nice!) and null out NA/VK and visa versa... of course if some LID is on same beam heading and it is kinda wide you are a tad out of luck). I found one man 'could' take a 4 sq, put it up, use it, take it down and it is all possible. sub 10kg for the lot, and not a £1500 price tag too BTW you do NOT need an estate ... first one I made at home was a 17m version (everything scales!) using T50-6 so it will fit in a domestic garden. qrp you can just point to point the whole controller too, don't need a PCB, I think my first was in an altoid tin. The box I waved in the talk (just like the ones you will see on this blog) was a 1KW capable thing. The 'HEAVY BIT' is the 1/4 wave matching lines from controller/switch box to the separate antennas as they HAVE to be made right and all 4 antennas HAVE to be identical and resonant where you want to operate. BTW I don't believe in tuners, why waste 50% of the RF in the tuner?
Use RG179 (not RG174) for the matching sections if build this qrp but watch out for cable being trod on. Why I started using WHITE SAT/TV coax as more visible (albeit heavier).