If you are editing HD footage than 4 gigs of RAM is too low.
Here are the listed minimum system requirements for Shotcut from the FAQ of the website: https://shotcut.org/FAQ/#what-are-the-minimum-system-requirements
You can try using the Preview Scaling feature which is found under settings. That helps reduce lag that may occur when using filters but it may not be enough if you have that little amount of RAM.
There is a proxy feature that is planned for Shotcut that would be great for users with low end computers but there is no definite date of debut. A forum member did make an unofficial proxy tool here but unfortunately it’s only for Windows not MacOS: https://forum.shotcut.org/t/shotcut-proxy-videos-auto-creation-auto-replace-tool-for-fast-smooth-editing/14519