Pull insulated flex duct over rigid duct
Going from R-4 to R-8 isn't a very big difference, and I doubt your plan would result in measurable savings, even if it were feasible. And it's not really feasible, since pulling flex duct around rigid duct is going to be nigh-impossible without removing and reinstalling the rigid duct and ripping the inner plastic liner of the rigid duct to pieces.

There are better solutions for the problem of ducts in an attic. Easy and cheap solutions:


Spray foam new insulation around ducts. This would give them R-15+, depending on thickness, and with a perfect air barrier, too
Wrap the insulated ducts in even more insulation
Bury the ducts in a new layer of cellulose covering the entire attic floor (easiest if they're resting on the attic floor)


Hard and expensive solutions:


Spray foam the entire attic at the roofline to bring it into the conditioned space of the house
Build entirely new ducts within the conditioned space of the house, possible hidden in new chases and bulkheads