It's interesting that the numbers 4 and 7 are often read using with kunyomi (よん, なな), even in many cases where the other numbers (1,2,3,5,6,8,9,10) would be read with onyomi.
For example, 34 is read さんじゅうよん (though さんじゅうし is another acceptable variant, I think?), but 35 would never be read さんじゅういつ.
What is the reason behind the use of よん/なな instead of し/しち ? That is, is there any historical or etymological reason, and why does this phenomenon only happen with 4 and 7 and not the other numbers?
One often cited reason (for 4) is that し is also the pronunciation of 死 (death). But this doesn't seem convincing, since し is a widely used sound in Japanese, and the meaning should be clear from context. Besides, し is used as a reading for 四, it's just that よん is also used.
Closely related is this question, but that is mainly about usage and does not cover the history and origin of this practice.