Ok, I'll post this again since your search button is broken.
IMR 4350 and H4350 ARE NOT THE SAME POWDER OR ANYTHING LIKE THE SAME. Only the numbers are in common. IMR4350 is substantially faster burning. IMR4350 powder DOES NOT contain a copper remover. You're thinking about the new Enduron line of IMR powders, not their legacy powders.
H4350 is very temp stable. IMR4350 is not.
If you get 1 granule of IMR4350 in a load of H4350 you'll get to throw that piece of brass away when the primer falls out and you might get to throw the extractor or the bolt away too. DO NOT MIX EVEN 1 GRANULE OF IMR4350 INTO AN H4350 LOAD. Ask me how I know.
An IMR4350 load will produce higher peak pressures than an otherwise identical H4350 load. I have successfully substituted IMR for H by dropping 1-1.5 grains per 40 grains of total charge weight. I do not recommend being as flippant as I was. Probably start 2 grains lower with IMR4350.
They really are different. Just because one CAN work in the loads the other works best in doesn't mean it's a good idea. High peak pressures and low velocities are the rewards for using too fast a powder.