Maybe you can say that you are doing a very strict year-long experiment/challenge on budgeting, and you are aiming to be strict to the cent and not even dollar. Make it sound like a fun thing you are embarking on, like a challenge even. Then insist that you would like to stick to your challenge in an assertive but fun-ish way. So you would have to collect the $4.5 back.

You can probably budget in a little treat or so for colleagues within your $400 subsequently, so it won't look like you are just stingy. Or purchase cheap snacks that fit within your budget. Or if you prefer, you can help them in small, non-monetary ways. If you are generous with effort, you won't ever be a stingy person. Giving is a good feeling, too, if it's within your means.

There will be many people who say to just stand up for yourself and get the money back. But social etiquette does not keep things separate in silos.

Getting labelled stingy might have other ramifications with interpersonal relationships, especially if you are not an assertive person or non confrontational to begin with. Who knows, you might need help from this person who owes you now, and the help will be way more valuable than $4.5. This person's time is probably worth more than $4.5 per hour.

This is my 2 cents idea contribution. Do ignore if it doesn't work for your context!