Daniel from Bolney CEP School wrote saying the following and included a picture:
I found out that you had to start with all four colours on one face. Then I made sure I worked diagonally with the other colours. This meant that all faces could show all four colours.
Julian from the British School Manila in the Philippines sent in the following solution:
Top face:
R Y
G B
Left face:
R G
Y B
Right face:
Y B
R G
Bottom face:
(imagine you could see through the top)
Y G
B R
Back face:
B R
G Y
Thank you and well done! If you thought it was impossible, have a think about those solutions above.