As expected no problem uplinking my Quanta LB4M to the LB6M using 1G RJ45 - stuff connected via 1G RJ45 to my LB4M can contact stuff connected via 10G to my LB6M, when the two switches are interconnected with 1G RJ45 (I used port 25 on the LB6M).
Default config on both switches.
@cstone94 if you're still having problems, give some more details on what you're doing. Are you doing anything more complicated, like VLANs? If not, maybe the first thing you should do is reset the LB6M to default - perhaps it's got some config on there from the previous owner?