As others note above, one approach is to use Hub Connect or Hub Mesh. If using Hub Mesh, the two Hubitat Hubs have to be on the same firmware revision. If that’s the route you go, understand that each hub is managing its own Z-Wave mesh, and the two meshes are separate.
If, instead, you choose the migration route using Bruce Ravenel’s guide for manual migration, be aware that you have two parallel meshes. As you build the new (C-7) mesh and move each device from one hub/mesh to another, the C-7 mesh starts out weak and grows stronger as you move devices to its mesh, while the C-4 mesh starts out strong and grows weaker as you remove devices from the C-4. At some point the C-7 mesh starts working well, and, at some point, the C-4 mesh starts working poorly.
Best to build out from the C-7 in quadrants/slices to the perimeter so that the C-7 mesh comes up strong in that direction.
Or, you can rip the Band-Aid off, remove everything from the C-4, start building out from the center.