It woulds be better to replace R-4 ductboard with R-6 ductboard or R-8
ductboard. Properly installed it is very effective at sealing out air
leaks. I would rather have ductboatd than sheet metal as it is
thermally more efficient in most installations.
With wrapped metal duct, you must seal every seam in the metal duct,
then wrap it with insulation and seal every seam in the insulation.
This includes both transverse and longitudinal seams in both the metal
and the wrap. ductboard only needs sealed once, using UL-181 A-P tape
; or duct mastic & glass fabric tape. DO NOT use duct tape.
NOTE, this is NOT a DIY job.
If rou put more insulation over ductboard, condensation can occur
inside the insulation, depending on conditions and where the dew point
occurs. Insulayion is not an air barrier. I do not recommend it. BTW,
due to its density, ductboard IS an air barrier.
As for reflectix, new data seems to support its claims. I did not
believe it at first, but some very knowledgable people I know say it is
good stuff, which agrees with my experience.