PGmc strong class 4 verb

Question

Why is PGmc *brekaną strong class 4 verb (not class 5)?
It doesn't have the ablaut vowel followed by a sonorant (m, n, l, r).

Answer 1

Presumably for the same reason that similarly structured roots can also be found in class 3: metathesis.

The Wikipedia article on strong Germanic verbs notes:

In class 3, there are also a few cases where the vowel is followed, at least in Proto-Germanic, by two consonants, neither of which is a nasal or a liquid.[3] Examples: *brestaną "to burst", *þreskaną "to thresh" *fehtaną "to fight". All but one have a nasal or a liquid in front of the vowel. This will have become syllabic and resulted regularly in u before the nasal or liquid, which was then metathesised on the analogy of the remaining principle parts. E.g. part 3 of *brestaną will have been *bʰr̥st- > *burst-, reformed to *brust-.

^{[Note 3] Ringe, Don. 2006. A Linguistic History of English. From Proto-Indo-European to Proto-Germanic, pp. 226, 243.}

The same applies in class 4: the zero-grade of Proto-Indo-European *bʰreg- was *bʰr̥g-, which by regular sound change would, and probably did, yield Proto-Germanic *burk-. But since the three other principal forms of the verb all (regularly) retained *brVk in PG (*brek-, *brok-, *brēk-), the fourth one – which appeared only in the past participle – was reworked to *bruk- as well, to match the others.