according to this Forum the cheapest AMD GPU card for 4k viewing is the RX-560 : Good (inexpensive) AMD card for 4k (no gaming). - Ars Technica OpenForum .
Some more information about the RX 550 & RX 560 : AMD’s Radeon™ RX 550 and Radeon™ RX 560 | AMD Partner Hub
From above link on RX 550 & RX 560:
Radeon™ RX 550 and Radeon™ RX 560: Enjoy Gaming At Its Best
Both the Radeon™ RX 550 and Radeon™ RX 560 graphics cards come with features built for gamers, content consumers, and content creators. Optimized for streaming, recording, and sharing gameplay, AMD's Polaris-based GPUs ship ready to encode and decode at up to 4K 60fps using H.265 right out of the box.4
When buying a RX 560 be careful, there are several versions of this card. The other versions are not as powerful as the original RX 560.