Ivan,
Warp 4 with FP 6 is even older. The latest public fixpak for Warp 4 is FP15 and DDPak 2 (Device Driver fixpak). There were additional FP's released for Warp 4 for those with a SWC or other type of paid subscription.
Warp 4 + FP15 + DDPak 2 brings you up to Warp 4.50 (legally), but the non-CP version (the CP version was the SWC version / update, which essentially was a client version of WSeB - the base OS without the extra "server" stuff)
The big changes with the CP version of Warp 4 (what we know as 4.52) is the addition of LVM and JFS. There were a number of other things (bug fixes, device driver updates, updated installer so it wouldn't crap out on AMD and then Intel 64-bit processors, etc). It also came with the full 32 bit TCP/IP stack (although that can be added to Warp 4).
As for eCS, the big updates are a new installer, device drivers to make installation easier, with version 2 - bootable JFS, and additonal software to make the overall experience an easier one.
As far as I am aware, Mensys does not have direct access to OS/2 source code, other then code which IBM has themselves made available (such as code in the Device Driver / Development Tools, etc). Some code IBM has released to open source (JFS for example). As a result bugs which require direct access to OS/2 source code to fix still go unfixed. However things which can be fixed by hooking into existing code or replacing existing code with alternate code (such as device drivers, the work that has been done to get ACPI working, etc) - that's seen attention.