And if at some point the character only levels as a mage it would roll a d4 for each level it gains above the fighter ones.
This optional rule has the side effect of having more HP than the standard rule:
A F5/MU9 in the standard rule will have its HP as stated above: 5d8 vs 9d4 (5-40 vs 9-36)
Same character with the optional rule will have 5d8+4d4 (9-56 range, 32,5 average).