Quote - I am having trouble scaling cars up or down to match the size of V4.

Which version of Poser are you using? Are the cars figures or props? What are you doing when you are "having trouble scaling cars up or down"? What do you expect to happen when you do what you do? What actually happens?

Be precise, eg:

"I select the Body actor of the car and turn the 'scale' dial. I expected this to increase the overall size of the car, but when I release the dial its value returns to 100%."

Saying "I am having trouble scaling cars up or down to match the size of V4", is like writing to an agony-aunt and saying "I'm having problems with my love life. Any suggestions?". Without knowing what you are doing, and what is happening when you do it, it is very hard to guess what the problem you are having is.

Please don't take offence at this, I'm only trying to get a better understanding of the situation. The only facts I have so far are that you are using some unspecified   method, in an attempt to change the scale of some unspecified item. It's not much to go on.