Hello Alan,
Welcome to Communities.
There are various configurations of 'UCS C240 M4' on the ReadyNode HCL - please have a look and indicate which one has the same hardware specs as yours (specifically controller, Cache-tier and Capacity-tier devices):
vmware.com/resources/compatibility/search.php?deviceCategory=vsan&details=1&vsan_type=vsanreadynode&keyword=UCS%20C240%20M4&page=1&display_interval=10&sortColumn=Partner&sortOrder=Asc
But, yes I can see that there are variants stating supported for 6.7 U1/U2 but none for 6.7 U3 (whereas M5 versions are listed for 6.7 U3 and 7.0) - whether this implies Cisco will no longer support them as 'ReadyNodes' on 6.7 U3 or later is a question best asked of Cisco (and/or the party you purchased these servers from).
That being said, all of the listed components - controller, Cache-tier and Capacity-tier devices - I can see from the models I checked (including the drives that have reached 'End of Sale') are on the HCL for 6.7 U3 and most 7.0. So from a VMware-supported perspective it is fine, but to get into the nuances of whether it is still considered a 'ReadyNode' or not (and thus things like SAS-Expander support) is beyond my station.
Bob