With a 2-core processor and only 4GB of memory your system is woefully underpowered for 4K video processing.
From the FAQ: Shotcut - Frequently Asked Questions
What are the minimum system requirements?
Operating system: 64-bit Windows 7 - 10, Apple macOS 10.10 - 10.15, or 64-bit Linux with at least glibc 2.27.
CPU: x86-64 Intel or AMD; at least one 2 GHz core for SD, 4 cores for HD, and 8 cores for 4K.
GPU: OpenGL 2.0 that works correctly and is compatible. On Windows, you can also use a card with good, compatible DirectX 9 or 11 drivers. We do not have a list.
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.