. . . . If I want to install MM to 4 pcs, do I have to purchase 4 copies? . . . .
For personal use you can install on as many PC's as you have, however only 2 can be activated at any one time and only used by you.
If there are multiple users and/or for use in a business, training/education, commercial environment then I would suggest you contact Magix by email to infoservice@magix.net and enquire about multi seat licencing.