A vs Per
When one must use "per" instead of "a". For example, which of these sentences is right?

They all usually slept for just four hours a night.
They all usually slept for just four hours per night.

Question

A vs Per
When one must use "per" instead of "a". For example, which of these sentences is right?

They all usually slept for just four hours a night. 
They all usually slept for just four hours per night.

Answer

In many cases they can be interchangeable.

'Per' is used more where there is some kind of calculation involved, especially in business or scientific contexts.

'A' is more common in everyday contexts.

Answer

For me, "per" is used when it comes to price, speed or any other ratio. $10 per night, 20 miles per hour.
I would put "a" here. "They all usually slept just four hours a night". Without "for"

Answer

This is a difficult one and I don't know any rules governing their usage.  To my North American ears, both sentences sound OK for the spoken language, but, I would use 'per' when writing.  Also,  "what is the price per square foot?" is correct but "what is the price a square foot?" sounds horrible. I hope this helps.

Answer

a = per