A few days ago the One UI 4 beta started rolling out to Galaxy Z Fold3 and Z Flip3 owners in South Korea, now units in the US are getting it as well. As usual, you need to be signed up for the beta program, which you can do through the Samsung Members app.
Note that only unlocked phones can join in (that was also the case in Korea), so if you have a carrier unit you may have to wait for the stable version. Which may be a good idea anyway, since beta software isn’t immune from issues.
Samsung Galaxy Z Fold3 and Z Flip3 units in the US can now join the One UI 4 beta…