4 CPUs of 20 (NUMA node 0) Pegged at 100%
So you've got 5 'stacked' instances on a single Windows server. You haven't said exactly how many sockets/CPUs are available and how much memory though. I like setting affinity for each instance in such cases, even if I decide to have CPUs overlapping in the struggle to balance the overall CPU load (depends on each instance's load).

Any instance with more than 4 CPUs could use an explicit DOP setting in my experience - rarely over '4' in stacked cases like yours. Don't forget to set 'Cost threshold of Parallelism' for each instance to something reasonable (50?) to avoid excessive parallelism  - in your case this is even more important.

Remember that the memory left "for the OS" should be more now, since you have to account for the footprint of each instance (on top of SSIS etc). Check in SQL Config Mgr if SSAS is also running and adjust its 'max memory' accordingly, by default it goes for 80% of the whole server memory (!)

Also maybe worth taking away and 'Lock pages in memory' rights of the SQL Service account(s) so that the OS can breath and do its job better (if it pages, 
everyone suffers!). Also good practice is to set some reasonable 'min memory' for each instance.

I think running sp_blitz and sp_blitz_first on each instance would give you some quick pointers on more pressing issues.

You may also want to monitor some windows permon counters like 'available memory' and 'working set' for each of the processes running there in case you find particular times of the day/night when the server is suffering.