depending on what CPU you want to use .. here is a guide that you might deem useful https://docs.google.com/spreadsheet...TJFqqVxdCR9daIVNyMatydkpFA/edit#gid=611478281
(no B550's on there though)
As pendragon1 stated, B450/X470 mobo's would work for you just fine .. I run a 3600x w/ 2 x 16GB 3000mhz DDR4 sticks on an ASRock B350 Fatal1ty Gaming ITX/ac for my daughters rig if you want to go super budget and don't plan on getting a 4000 series Ryzen proc .. threw in an RX 570 4GB vid card and it's great for 1080p gaming (Destiny 2,Gear 5, Fortnite..etc .. )
You have good options, the B550 M Steel Legend was listed just around 150$ which is a good price for that board.
The PCIe 4.0 will become more usefull once you see more storage devices taking advantage..