Had the a similar problem with the V3 to V4 upgrade, V4 refused to accept my V3 serial as valid and my V4 upgrade as valid.
The workaround
1: install V3 in whatever folder as long as it's not the one you really want to use for V4. Serial for V3 is accepted
2: install V4 in the same folder where you just installed V3 , lo and behold your V4 serial is accepted
3: install V4 again but now in the folder where you wanted it in the first place , ofcourse your V4 serial is still valid

4: delete the folder where you installed VUE in step 1 & 2 , do not uninstall because that will remove the last install and you'll have to start from scratch again.

This trick could work for V4Pro to V5 sidegrade too :)