I got mine to work in the end, as follow:

* all nodes HAVE to be the same WSFC (as per documentation). cannot add a node into a WSFC if its already part of another.

* no magical 'merging' of 2x separate WSFCs

* pre-validate your 2x DC1 nodes, including storage. then pre-validate your 2x DC2 nodes, including storage. when all good, then create your 4x node cluster, and run validation, excluding storage.

* from there its just a matter of setting possible-owners on resources

* your 2x FCIs should have the same drive letters and paths to make AO easier to manage. bringing them into the same WSFC is not a problem, but while they are under the Available Storage default resource group you can obviously not assign the same drive letter twice. So at this stage you create your Roles in the WSFC, one for each FCI. First you bring all the Available Storage online, say on node1. half of them will stay offline, because they can physically only connect to node3 & 4. you create your Role and assign those disks to the new role. This takes them out of Available Storage, so you can re-use drive letters again before creating your Role2, for your FCI2

* now you have 2 Roles/Resource groups ready, so during install of Clustered SQL you just select this pre-staged Role.

* when both FCIs are installed, make sure possible-owners are all correct on all resources. (AG group create will actually fail/warn if some is incorrect)

* create AG & Listener and you're good to go!