Is it okay to mount EP422 in the front while keeping EP20 in the rear?

As long as you keep the newer tires in the rear and you keep track that the same tires are on the same axle when you do rotations, you're fine.

 

no, the newer, better tires (EP422 in your case) should be in the back to ovoid oversteer and 180 degree type slide.
every competent tire shop will refuse to put new tires in the front only.
this is why regular tire rotations are recommended to avoid it.

 

OP, I've read through your post about 3 times, can't understand: why aren't you replacing all four tires at once?

 

Gotcha. I tend to stick with EP20 for replacement then. Last time I purchased those, through Costco, it was a similar story: asked for EP20, they said none were in stock but would EP422 be ok. I almost caved, but then called them back and asked if they could order in EP20. A week or so and they were in.

 

I just had a similar experience (not at Costco)....EP20's were ordered and EP422's substituted instead without my knowledge or agreement. I told them I want to stick with what worked for me and that was the EP20's. I had great luck with the EP20's that came on my 2011 Prius - ~56,000 miles on them. I could have gotten more, but since I switch to winter tires, the tread was low enough that it wasn't worth keeping them and paying to have them remounted and balanced since I would need new all season tires shortly thereafter. Apparently the EP20's are on "backorder", which I hope doesn't really mean that the tires have been rotting in a warehouse for years. I think the EP422 is a better rated tire, but for my particular driving habits, I'm sticking with what worked for me.