HD 4850 is a 1000 gflops gpu but wii u only has 176 gflops. Architecture may be similar but actual performance far less on wii u. Switch will be easily far superior even if the switch spec is at the lower end of expectations.
gflops has alot less to do with real world performance then people are expecting. Its a very, very theoretical based number.
According to this website its 352 gflops btw. https://www.techpowerup.com/gpudb/1903/wii-u-gpu and in terms of graphical performance the wii u is a pretty good match to current modern arm chipsets.
Dont expect much better graphics from the switch. Like I said, its even very possible that the performance is lower when nintendo is going for an cheaper priece. Like the 3ds also has less performance then the wii.
But if you want to look at gflops performance in comparison, the 4650 is 320 gflops, so similar to the wii u.
It achieves 75 FPS in trex 1080p offscreen benchmark. Still very comparable to the 115 fps from the (always plugged in) shield console and its tegra x1.
The Pixel c (tablet that uses tegra x1 - so not plugged in) achieves 80 FPS as comparison.