Regex: Find only full words? [closed]

I have different titles and try to extract the models.

The models are mostly one letter and two numbers or one letter, two numbers and one letter.

I would like to consider only words that have no more than 3 or 4 digits.

1. BMW 535d Touring F11N
2. BMW M550dX Touring F11
3. BMW 520d Touring G31

Regex: [A-Za-z]{1}[0-9]{2}

Result:

1. F11 => Must be F11N
2. M55, F11 => Must be F11
3. G31 => correct

How can I prepare the regex so that only words with 4 characters are considered?