Looking over my targets, it seems that accuracy was better with Unique than Blue Dot. Looking over Harry Jones chart on main spring, I really dont see any major difference in main spring strengths. I think the load was just hot enough to be able to function both pistols, I would GUESS that the 4" got hammered a little more from recoil. Looking over powder burning charts your SR 4756 would be about 1/2 way between Unique and Blue dot so it seems like it would be a good powder for the 9mm. Here is one of the better targets.
<a href="http://members.rennlist.com/lugerman/DLTarget.jpg" target="_fullview"><img src="http://members.rennlist.com/lugerman/DLTarget.jpg" width="400" alt="Click for fullsize image" /></a>