I was at least partially responsible for EntMod v4 coming into existence. I bugged DF one day several years ago saying that v3 caused some newer HL mods and Sven Coop 5 to crash. He made v4 specifically for those games. I believe Hook Mod v6 was also done for that reason as well, but I never used that plugin.
I didn't realize DF took down adminop.net for whatever reason, probably tired of paying for the domain or something. I don't have Hook Mod, but I do have EntMod v4, both the Windows and Linux binaries. One thing to note is that EntMod v4 should only be used on SC5 or newer HL mods because it will cause older HL mods to crash or behave strangely, just stay with EntMod v3. There are no functionality differences between the two, so there's no reason to run v4 unless you actually need it.
You can download it from my archive site:
If you need an alternative to Hook Mod, you can use the GHW GHR plugin for AMXModX.
I'll bug DF again and see if he can upload HM v6 somewhere for people to download.