I've built openldap 2.4 packages for EL4 and EL5. Since many base packages link against openldap, I include an appropriate compat-openldap package for each --
EL5 has 2.3.27, and EL4 has 2.2.13. That way, all of the base packages which link against openldap will still work, but you can run the newer version of slapd.
The 2.4 release announcement outlines many of the features and can be found here:
http://www.openldap.org/software/release/announce.html
The packages/spec files can be found here:
http://staff.osuosl.org/~jeff/openldap/
Checksums for the rpms are here signed with my gpg key:
http://staff.osuosl.org/~jeff/openldap/checksums.txt
Any questions/feedback welcome :) I've tested the EL5 clients/server/compat out and it seems OK, but I'd appreciate if someone could test on EL4.