In his editing for the 1837 edition, Joseph Smith removed the word what that heads a comparative nominal clause. There are two other passages where what heads a comparative nominal clause, but in these cases the usage is different:
In both these cases, the what serves as the direct object for the verb in the relative clause (“what I said” and “what he hath gat”). In Mosiah 4:27, on the other hand, there is already a direct object for the verb in the relative clause (namely, strength), thus the strangeness of the original phraseology (“than what he hath strength”). Nonetheless, the critical text will restore the what in Mosiah 4:27 since it appears to be intended. For another example of a passage where Joseph Smith deleted a strange-sounding complement after than, see under Mormon 9:31.
Summary: Restore the what that Joseph Smith deleted from the comparative nominal clause in Mosiah 4:27 (“it is not requisite that a man should run faster than what he hath strength”).