Of course your theory is quite sound, but I will point out that in practice most 4k streaming content uses an HEVC codec, while most 1080p streaming content use

Of course your theory is quite sound, but I will point out that in practice most 4k streaming content uses an HEVC codec, while most 1080p streaming content uses an AVC codec, so you'll likely have much better results on your data budget with the 4k signal even if significantly downsampled to your display.