I've issues running 4GB Fallout New Vegas Updated with ENB (I'm using The ENB of the Apocalypse). I'm using Injector version of the ENB (should I use the wrapper version instead?) as the mod author guides in the description. I also edited enbinjector.ini to include the line Fnv4gb.exe at the top.
When I try to launch to game (fnv4gb.exe) I see this error: "failed to write to memory of child process - access is denied"/"steam_api.dll missing". Both fnv4gb.exe and fnv4gb_helper.dll are located in the main New Vegas folder with all ENB files. I've the latest stable (4.5) version of the NVSE.

Also, do I really have to always run enbinjector.exe before starting the game?
ENB of the Apocalypse guide (the first post): http://www.nexusmods.com/newvegas/mods/50782/?tab=4&&navtag=http%3A%2F%2Fwww.nexusmods.com%2Fnewvegas%2Fajax%2Fcomments%2F%3Fmod_id%3D50782%26page%3D1%26sort%3DDESC%26pid%3D0%26thread_id%3D1027458&pUp=1

Do you by chance have your game installed in your Program Files folder? That could be your problem, the security systems in place may be interfering with your ENB installation.

No, the location is D:\SteamLibrary\SteamApps\common\Fallout New Vegas.