> 4x4: Not enough to draw "E", "M" or "W" properly.
However, 5x5 isn't enough to draw "e" properly if you also want lowercase letters to have less height than uppercase, so you need at least 6 vertical pixels. And then that isn't enough to draw any character with a descender properly, so you need at least 7 vertical pixels (technically you should have 8 in order to allow "g" and "y" to have a distinct horizontal descender while still sitting on the baseline, but this is probably an acceptable compromise). And remember that in practice this means you will still need at least 8x6 pixels to draw each character, to allow for a visible gap between letters below and beside them.
However, 5x5 isn't enough to draw "e" properly if you also want lowercase letters to have less height than uppercase, so you need at least 6 vertical pixels. And then that isn't enough to draw any character with a descender properly, so you need at least 7 vertical pixels (technically you should have 8 in order to allow "g" and "y" to have a distinct horizontal descender while still sitting on the baseline, but this is probably an acceptable compromise). And remember that in practice this means you will still need at least 8x6 pixels to draw each character, to allow for a visible gap between letters below and beside them.