To clarify so we understand the situation, did you have Mageia 1 installed and just installed Mageia 4?
Are you trying to boot with Grub Legacy from Mageia 1?
What does your menu.lst entry for Mageia 4 look like? Have you tried chainloading it?
when I'm booted to Mageia 1 it won't mount the partition I installed 4 on
Post the command you use to mount and also what the output is when it fails.