Rapid PVST is supported in Sup1/2 in CAT4k with 7.5(1) and later
PVRST+ is in Early Field Trials now for Cat 4k Sup3/4 IOS. I have tested interoperability with 3550 12.1(13)EA1a and it works well. Less than 1 second failover/convergence.
Rapid PVSTP is currently not supported in Sup3/4 but will be in an IOS release expected later this year. RSTP/MST is already supported
There are no new software features in 12.1.19E
You'll need to wait for 12.1.19EW1