2018 MacBook Pro (w/ Touch Bar): Are all 4 thunderbolt/USB-C ports capable of supporting a monitor?
According to Apple's specifications on this MacBook Pro all four ports support video:

 Charging and Expansion

 Four Thunderbolt 3 (USB-C) ports with support for:

 Charging 
 DisplayPort 
 Thunderbolt (up to 40 Gbps) 
 USB 3.1 Gen 2 (up to 10 Gbps)


If two of the ports are not supporting video you could look at the cabling (try another cable, etc.) and monitor compatibility (Thunderbolt 3?). Failing that take it to an Apple store and have them test it.