Substantially more work. However, what people do not realize about these "toolkit ports", whether within a family (GTK 3->4) or across them (Qt -> GTK), is tha

Substantially more work. However, what people do not realize about these "toolkit ports", whether within a family (GTK 3->4) or across them (Qt -> GTK), is that the reason they take so long is that it is almost impossible, when having to go through the vast majority of the codebase and change stuff everywhere, to avoid the realization and/or temptation that there are better (or at least different) ways to do things. If you could suppress that inclination (and its not clear that doing so is a good idea), and simply do as close to a 1:1 replacement where required (which for cross-family ports is everywhere), the port could be a lot faster. But it is extremely hard to resist that, because the non 1:1 replacement stuff just appears so right.