if you're going to be using 4b for mixup, you should definitely be using the 6a link or underdash 2dd...the damage pay off is just too high compared to the CT combo lol
on the rare occasions that I do use 4b, I land 4b > 6a link consistently during matches as well. the hardest part is the 6a, and after that it's pretty much smooth sailing.
the only time it's ridiculously hard to link is when you're RIGHT NEXT to the opponent when you start the 4b...I have never been able to land kure after that, even with 2149d. spacing like that won't happen that often though unless you're doing meaty 4b on wakeup or something.
I don't know if it'll work for you guys, but I input 4 right before I do 6a (while the opponent is falling). just a small nudge, then 6a. helps out tremendously with the timing, but that's me. seemed to help out another friend I suggested it too as well.
