Answer:
very less
Step-by-step explanation:
When rolling a fair 6 sided die 5 times there are 6^5 possible combinations.
Of those, there are 5 ways to roll exactly four sixes, and each of those ways have 5 possible results on the die which has not rolled a six, plus there is 1 way to roll five sixes. 5*5+1=26 combinations with at least four sixes.
Now, take the number of results which fit your conditions, divide by the number of possible results and you get your answer: 26/7776 which can be reduced to 13/3338, a bit less than 0.4%