I've been trying to understand the how strong verbs in Germanic languages work, and reading the Wikipedia article I understand that class 4 strong verbs originated from, in PIE, vowel + a sonorant (m, n, r, or l).
But there seems to be many words whose vowel is not followed by m, n, r, or l but are of class 4. For example, German sprechen and Dutch spreken and English speak is class 4, yet in Old English and Old Dutch the same word seems to be a class 5.
In the article it is noted that
Get, speak, tread and weave were originally of class 5, whereas swear was originally class 6.
How did these verbs "switch" class? And how can a verb like speak become a class 4 verb?