Viewing moon craters

Hello all, i received a telescope as a gift and am interested
in pursuing it further. It is F=900mm and D=114mm. We were
viewing the moon using a 12.5mm lens and got a good clear
shot. However, when trying to use the 4mm lens, it became
just a blur. Would 4mm be too powerful for viewing the moon?

I am a complete newbie, but according to the picture on
the box i should be able to focus in on individual craters.
Is this reasonable given the equipment that i have?