I haven't looked at R4 in a long time, but you should be able to use the menu Format> Dimension Styles... to bring up a dialog. The usual selection is Default-ANSI unless you've set it to something else.
Create a new dimension style by clicking new, providing a new name, and setting the options the way you want them to be. Especially the units you want to use. Then, Save it.
Now go to Format > Standards > Dimension Styles tab and set the style to the one you just made. It seems to me there is a dimension style called ANSI MM available already. If so, you can adjust it to be the way you want.
Bear in mind... since I haven't looked at R4 in a long time its possible these options are not all in R4, and are in R5 instead. My apologies if that proves to be the case.
Bogie