This is feedback on my experience upgrading to 4.0. After upgrading, my apache is dying at least 5 times a day, causing all sites on the server to be unreachable. I never had this even once in the last 5 years.

Forum has 2 million posts with 30,000 page views per day, which is not that much for a very powerful dedicated server with only a few other less active sites on it.

Disabling the one minute cron dropped the load down, though still slightly higher than before. I will watch it today and see if the apache problems come back or not.

Are the background processes still running or have they finished?
If they're still running, I would suggest to activate the cron feature ( ACP => System => Advanced Configuration ) which will speed this up

The background processes were still running. I had already activated the cron setting.

It looks like the crons were what killed my server. I will test it without the crons running for a day.

So far load is looking very normal now that the cron was disabled. But I will have to get the background processes completed slowly without over burdening the server, maybe with a bigger time gap, or maybe manually.