The 4GB of memory is almost certainly the issue. The FAQ states that the minimum requirements regarding memory are:
RAM: At least 4 GB for SD, 8 GB for HD, and 16 GB for 4K.
NOTE: If your computer is at the lower end of CPU and RAM requirements, you should use both the Preview Scaling and Proxy features to help reduce preview lag.
Try rebooting your system and running Shotcut without anything else open. Also you might want to look at the Task Manager->Performance-MEM utility to see if you come close to exceeding your memory limit and start paging.