No, you can also have 8 MB of texture in your map. But 4 MB is more then enough. Even for big maps. And if you DO go over that limit, you can use this compile parameter to override the default amount of MB's for textures: -texdata 8192 (8 MB).
And you need to add that compile parameter to all four tools!
I recommend a front end compiler. Very easy to use. Nem's Batch Compiler is one of those, but using it might be a little complicated at first, so you can also go for the Half-Life Compilation Controller, the one I've used since I began mapping, about three years ago. Get it
here.
It already has the texture date set to 8192 MB. It has most of the used compile parameters in the user interface.