如何为preg_match设置最小字符串长度

I am trying to validate user input in a php script. I want the var to be only lower case letters with a minimum length of four characters. I tried the following:

preg_match('/^[a-z]{4}+$/', $my_var)

But this only matches if the string is exactly 4 characters in length. How can you say at least 4 with no maximum?

preg_match('/^[a-z]{4,}+$/', $my_var)

{4,} will match strings of length 4 or more.