With one number cube, what is the probability of rolling a number less than or equal to 4?

4 in 6. or simplified to 2 thirds. 
this is because there are 6 possible outcomes, 1, 2, 3, 4, 5, and 6. 

four of them are equal to or less than 4. 

1, 2, 3, 4

so, four out of the six, or two of the three