annular discharge is the type of boosters the carby uses, these are normarlly used on a carby which is a bit too large. The annular boosters discharge more fuel on a weaker signal but at high revs normally cause a fat AFR. The best application of these boosters is on a Holley 1050 dominator used on a <400ci small block.
Square bore means the carby is a 4150 series carby and the secondary butterflys are the same size as the primarys.
4 corner idle means the carby has 4 idle screws, one for each barrel. This is designed for eniges with big cams.
This is a serious carby unless you are competent tuner I would not recomend this carby because there is a lot to fiddle with.
WHats your engine? WHy do you say its hard to tune? 4 corner idle just makes it alot nicer carby and your right it is better than a 2 idle carb on a big cam motor but it is also right at home on a small cam motor. Anulars arent always desined for the fact the carbs to big for the engine they areso good when an engine is in a large style car as they promote bottom end power
some of this might help.
i had a brand new 650 vac secondary holley on a modified clev a few years ago.i got annullar discharge boosters put on the primary side.they make a 650 cfm sort of like a 590 cfm as they are bigger than the original booster,so ultimate flow is less(because there is a bigger item in the way,).but responsiveness at all revs is much sharpe,the signal is stronger.they are well worth it,i guess these days you can get holleys with them standard,dont know.
i got mine fitted by high flow indution techniques,i think they are still around i was getting them to flow the carb and set it all up just right so got them to fit the annular boosters.
(i would reccomend h.i.t for any work for those in w.a,they explained patiently any questions i had about this stuff)
as for four corner idling i have no experience with it but im guessing you have idle adjustment on all four venturi,instead of just the primarys,but im just guessing,
isnt the '4 corner idle' an option on the 'double pumper' (all 4 open mechanically)? If so, probably better off with a vacuum secondary for driveability and fuel economy- unless you like to buy gas.
I think 'annular' kinda means 'around a circle' type fuel inlets- unlike standard where fuel shoots into center only of venturi.