Watch out folks as this update can be problematic on Skylake and later platforms. Update Lilu + WEG kexts to latest available versions and Clover to r5107 before initiating the update. If you cache your kexts from /L/E (or /S/L/E), remember to also update the kexts in the E/C/k/O of your EFI partition as the temporary update partition boots only with kexts injection. Faking a different CPUID should no longer be required but boot parameters -lilubetaall and/or -disablegfxfirmware may be so until the update has fully completed.
I've also noticed that 10.15.4 no longer supports XCPM on my Ivy Bridge laptop (Latitude E6230) so it's back to generated CPU power management SSDT + AICPUPM patch.
At the same time Safari 13.1 + Security Update 2020-002 were released for Mojave 10.14.6 and HIgh Sierra 10.13.6.
Warning to all those who run Catalina on officially unsupported graphics platforms (eg: 1st gen Intel HD, Intel HD3000, nVidia Tesla), the patches/tricks previously used to gain OpenGL-only graphics acceleration no longer work in 10.15.4. As such, stick to 10.15.3 for the time being!
Edit: April 8th, 2020
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Unsurprisingly, Apple issue a 10.15.4 Supplemental update today. This brings 10.15.4 to Build 19E287. Looks safe to install on all systems running Clover r5107. Had trouble on Latitude E6230 running with Clover r5108 (and reverted to r5107).