A 4GB drive is too small. The Windows 11 ISO file alone exceeds the capacity of a 4GB drive.
Microsoft’s official Media Creation Tool and ISO file require at least 8 GB of free space on the USB flash drive. However, a freshly created Windows 11 bootable USB will actually use about 5–7 GB, depending on the edition (Home, Pro, etc.) and included updates. windows 11 usb size
If you have a 16GB drive, you can easily repurpose it. However, you cannot put both installers on the same drive at the same time (unless you create a multi-boot drive, which is advanced). You will need to reformat it and switch the ISO files whenever you want to change versions. A 4GB drive is too small