4k would take a very long time to seam-carve and it might even appear to hang at times due to how long it can take to process. Just reducing by 25% would be something like 3'628'800 carving iterations! I have some optimization ideas though to get it to re-use work across frames which would drastically increase this speed, but it may take some time unfortunately.
Does it still work on smaller frames or at half/quarter res? Is it crashing or just hanging?