I’ve been using 3D-Blu-ray-Rental for a while (despite the name they have a wide selection of UHD discs as well). Their pricing looks better compared to the service you mentioned (the $8.99 tier includes 2 rentals whereas they’re an additional $1.99 each on the Rent4K service).
Positives - they have a nice selection, customer service is good, I’ve never received a bum disc.
Negatives - They’re not Netflix fast when it comes to shipping, and they seem to have a limited stock on newer releases, you may have to wait a few days for someone else to return a disc before it’s shipped to you.
Still, I’m happy with it, and I’ll stick with it until Netflix starts renting UHD discs.
Positives - they have a nice selection, customer service is good, I’ve never received a bum disc.
Negatives - They’re not Netflix fast when it comes to shipping, and they seem to have a limited stock on newer releases, you may have to wait a few days for someone else to return a disc before it’s shipped to you.
Still, I’m happy with it, and I’ll stick with it until Netflix starts renting UHD discs.