Is there a way to remedy this? and also, can I really only select four units?

Here's a utility (written in AutoHotkey) which adds click-to-move and a lot of other mouse/keyboard functionality:
https://community.pcgamingwiki.com/files/file/954-warcraft-mouse-helper/

And yes - you can only select four units at once. That was actually an intentional design decision because they thought being able to select a whole horde of units would make the game too easy.

Remember: this was one of the first RTS games, so a lot of the standards for the genre hadn't been established yet.