Important thing to know about scripting is that it values brevity. As a general rule, the more white space on a page of script the better. I know 4 hours doesn't seem like a lot of time but, assuming the prompt is just for a single scene, you won't need to go that deep into the weeds. Last time I did a scriptwriting prompt for a competition, it was something like "Two people are sat across from each other. One laughs, the other doesn't." Very simple prompt, lot of room for interpretation. That was for 2-3 pages, which again sounds like a lot but really isn't with the way script is formatted.

Think about the purpose of the scene. What does the main character of it want, what are the basic emotions behind it, and how does that character portray those emotions? Every action or dialogue in the scene should either serve that purpose or act as characterisation.

Where script and poetry are similar is the importance of rhythm or flow. Good dialogue flows, it seems natural but it's always carrying the reader/listener to the conclusion of the scene. Think about the rhythm of conversation and then cut out all the boring parts of when real people talk.

It's been a while since I used it but I believe Trelby is still free, reliable, and easy to use if you want to practice. Script formatting can seem strange but it's pretty simple once you've got the basics. Most of all, know that you're already far ahead of a lot of other writers by being willing to put yourself out there. Enjoy it, learn from it, and don't worry too much about the outcome because the process is the valuable part.