As was said in the other thread, if you have prime and aren't ordering over $100, this is a decent savings on a very endearing rocket. I'm trying hard to resist buying more Big Daddies since I still have a few unbuilt, and by the time I bought, say, 3 more of them, I could easily add my next order of Estes motors to the total and break the $100 free shipping threshold at AC Supply.
Of course!
4x29 should fit, actually. Just going off BMS dimensions since they're easy to find (but from experience, I believe them nearly identical to Estes, at least in 3":
ID of the 3" tube: 74.42mm
OD of heavy wall 29mm motor tube: 30.73mm
Ideal 4 into 1 circle packing diameter from
my favorite Wikipedia article: \(( 1 + \sqrt{2}) * OD =\) 74.19mm
Screw-on retainers won't work at that density of course.
Edit to add: the width of thrust rings might spoil the fun, now that I think about it.
Edit again: just measured an F42 thrust ring at 31.7mm. That would give a 4x cluster OD of 76.53mm, vs. a 3" OD of 76.2mm. Very tiny overhang. Might need to do considerable sanding where the motor tubes meet the body, but it could work.