Nausadh, i also have same understanding, like wearing both 4 mukhi and 5 mukhi are beneficial.
Generally one is supposed to wear rudraksha beads
in odd numbers, So possibly you can always mix these two beads in such mala.
For your reference to the reason, you can see that saraswati rudraksha mala
, contains a combination including 4 mukhi and 5 mukhi. This mala in such a combination is seen to work much better. So no doubt on such combination.
Also the recommendation on wearing rudraksha beads may differ from person to person. If someone is having good knowledge and memory power then possibly 4 mukhi won't be recommended in first advise. But wearing them does not make any issues.
Om Namah Shivaya