As an addendum as a language teacher:
There are two kinds of mistakes: "Slips" and "Errors"
A slip is when someone makes a mistake once, but knows the correct form, and uses it normally most of the time.
An error is when someone genuinely doesn't know the correct form and routinely makes the same mistake again and again.
As a teacher, it's important that you offer corrections for both when you can, but correcting slips is far less important than correcting errors.
Her teacher will not be able to differentiate between slips and errors until she gets to know your daughter better over a longer period of time, and even then, with very young children it can be difficult to tell. The teacher has to learn this info about every child in their class.
As a parent, you're right to ignore most slips in favour of building self esteem and confidence, but ignoring errors is just going to cause problems in the long run, so it's a balancing act between the two.
At any rate, she's 4. She has plenty of time to get it right. :)