Generally I tell people to apply one of two different Fixpacks, depending on what hardware they are running OS/2 on.
If you are running OS/2 Warp v4 on an Intel 486 or P60 derived chip, it is easy to apply Fixpack 12, and leave your system at this level. Fixpack 12 is very stable but does not include the new memory mapping model found in Fixpack 15.
I doubt that this new memory mapping model will affect you in any way.
FixPack 15, or FixPack 16 or 17 (SWC only)
If you are running OS/2 Warp v4 on an Intel Pentium class CPU, then this is the Fixpack for you, you can also apply this fixpack to a Intel 486 class CPU PC but you will need to replace the \os2\boot\clock01.sys file, with the copy you DID make of it before applying the fixpack. You may also need to replace the OS2DUMP file.
FixPack 13 started the progression to a unified Kernal for both the OS/2 Client, and Server versions, so Fixpack 13 and 14 did introduce some problems, Fixpack 15 appears to have fixed these problems, and for over 6 months I have found it very stable. After applying FixPack 13 and higher, your version will be reported as Warp v4.5.
Convenience Pack
If You have access to IBM's SoftWare Choice (SWC), I would recommend you upgrade to CP1 or higher if you have a Pentium III or higher.
The other option is to purchase eComStation, which is at the same build as IBM's Convenience Pack but also includes a lot of extras including Lotus Smartsuite.