Getting a little 4350 in a can of 4064 probably wouldn't hurt much- I would mark the can as 'plinking ammo only' and use it for slightly reduced loads just to be safe though. I did something similar a few years ago with 3031 and 4064- I couldn't remember what I had left in the powder measure a few days before but knew it was either 3031 or 4064 and ended up setting it aside and using 3031 load data for making plinking loads.

There's very good reason for not mixing powders even if you end up with a slower powder mixed in- the pressure of the powder and burn rate are connected- not sure how a slower powder might react if it were in a cartridge where the pressure spikes much faster than the slower powder would burning on its own. For this reason alone, I would reduce the loads out of that can of powder below your maximum worked up loads.