The procedure all depends on what redundancy your connected devices have. If they are mainly hypervisors and other devices that are multihomed, this can be done quite nicely. The more single-attached devices you have, the longer the downtime.
I guess you have already read this guide:
https://www.juniper.net/documentation/us/en/software/nce/nce-171-qfx-vc-upgade/topics/topic-map/nce-171-qfx-vc-upgrade.html
It is easier to handle 2 member VCs in this scenario. Perhaps splitting the 4 member VC into two VCs would be a good idea.
Another idea is to go for eVPN on the QFXes. This may require a license upgrade, but gives you superior flexibility and any (well, most) upgrade issues will become a memory only:
https://www.juniper.net/documentation/us/en/software/junos/evpn-vxlan/topics/example/evpn-vxlan-qfx5110-l3-vxlan-gateway-non-collapsed.html