Act 2: Gob -> ancient path gave me all 4 types of perfect star designs (also 3x emerald btw) and 3 types of flawless star designs except amethyst. (farmed with 303% mf without nv, 1000+ gobs) Not a single radiant design yet, probably drops in act3+ and ponys.
I think Magic Find has a big effect on plans dropping.
I used to farm Warrior's Rest with 300 MF and I'd get multiple plans an hour. In fact, I had back to back exalted Dread Shield plans drop in there. I don't think the rates are based on zone, but they have an ilvl so you will likely get the crappy 4 prop ones in Act 1 and you'll have better chances at the 6 prop ones in later Acts.
Just from Warriors Rest I would say I got 12 Exalted Dread Shield drops. Dread shield definitely dropped the most but I think I got all of the 4 prop armor drops at some point in Warrior's Rest. I also had a legendary pattern drop (the wizard wand) from Warrior's Rest.
To prove that 4 prop armors still drop in later Acts, I was farming Act 3 earlier this week and I got ANOTHER 4 prop dread shield. Those things love me.