i simply looked at the spec sheet for the mobo.
https://www.asrock.com/MB/AMD/B450 Pro4/index.asp#Specification
under storage it says
- 1 x Ultra M.2 Socket (M2_1), supports M Key type 2242/2260/2280 M.2 PCI Express module up to Gen3 x4 (32 Gb/s) (with Vermeer, Matisse, Cezanne, Renoir, Picasso, Summit Ridge, Raven Ridge and Pinnacle Ridge) or Gen3 x2 (16 Gb/s) (with Athlon series APU)**
- 1 x M.2 Socket (M2_2), supports M Key type 2230/2242/2260/2280/22110 M.2 SATA3 6.0 Gb/s module and M.2 PCI Express module up to Gen3 x2 (16 Gb/s)**
easy to see. 2 slots, 1 3.0 x 4 and the other 3.0 x 2 or sata.
no reason to doubt what is right there for all to see.