We've been running 5.1 SP4 now for 9 days, and so far there are no issues. The handset buzzing / echo complaints also seem to have ceased. No alarms yet. It took 25 days on SP3 to throw a weird alarm, so I'll be especially watchful of it around then.
Between SP3 and SP4 they changed 5 packages: Call Processing, Shared Libraries, Resource Command Handler, mtl-5000-5_1-release-32_SP_4, and US Voicemail. All of which sound pretty important, to me anyway.