If we go beyond ASCII, Unicode specifies 2x2 mosaics since ever (they were present in DEC terminals) and 2x3 mosaics (from Teletext and the TRS-80) since version 13. Some more enlightened terminals (such as VTE) implement those symbols without the need of font support.
Or you can use Braille to get 2x4 mosaics, but they usually look terrible.
Or you can use Braille to get 2x4 mosaics, but they usually look terrible.