Information of DebugLevel 4 and 5 is very helpful when investigating complex bugs or simply troubleshooting certain processes in Zabbix lacking user-visible error messaging. I cannot overestimate the usefulness of the runtime control of logging level on process type and individual process level. Unfortunately, on highly loaded environments it's not really an option. Especially when the issue being investigated is quite rare, unpredictable and can only be detected some time after inception. Enabling additional logging would slow down the system and produce too much information to analyse.
My suggestion is to introduce a cache for the most recent logs of all possible levels and a runtime control option to dump this temporary storage into logs on demand. Basically, same strategy is used in flight recorders with great success.