I have a PhD interview and I have to start off by talking about a research project I have been involved in. My problem is my fourth year project didnt seem like a full research project as my aim was to create a program to fit data to a model. It's been done before in several scientific packages and my program wasn't even able to do it well. My third year project gives me more to talk about and felt more like a research project.

Do you think it's a bad idea for me to present my 3rd year work over my 4th year work? Both my projects relate to the phd topic however my 4th year project relates to more aspects of the phd. Thanks in advance.

It should be perfectly fine to present your third year project instead of your 4th year if it is more relevant. The interviewers probably don't care what you present as long as long as you present it well and it somehow relevant to doing a PhD or your project.

Saying that, you might want to quickly mention that your 4th year project was highly technical focusing on..... and that you gained skills in these areas.... Just so that it doesn't look like you are not hiding anything and that it just wasn't research focused.

Goodluck!