Verified answer

i think さんぶん actually shoud be

さんぷん, and よんぶん should be

よんふん.

i don't really know why those things are the way they are.

probably it was originally all the same to begin with,

like, いちほん、にほん、さんほん、

but somehow ended up being little different due to

how better each sounded.

the laziest way is though...

to remember just

ひとつ

ふたつ

みっつ,etc.

and use it for most things.

of course this can't be used as a substitute

for something like いっぷん、にふん、さんぷん、etc,

but i use　ひとつ version for many things,

like when shopping,

i might say, like for some だんご or やきとり,

これひとつください

instead of これ一本ください.

It's not like English is consistent either. Say these words: though, enough, trough. Why is read the same as reed, and yet read is also the same as red?

Life often doesn't make sense. Language is a reflection of life. So it makes sense that languages often don't make sense.