The result of rolling a number cube 7 times is a 7-digit number composed of digits 1,2,3,4,5 and 6 so that digits can repeat. The total number of possibilities is 6^7.
The number of possibilities where 4 appears exactly two times is 5^5*(7!-6!/2). 5^5 is the number of 5-digits numbers composed of digits 1,2,3,5 and 6 so that digits can repeat. 7! is the number of permutations of digits 1,2,3,4,4,5 and 6. 6! is the number of permutations of digits 1,2,3,{4,4},5 and 6. We don't want to subtract all numbers where digits 4 appear side by side. That's why we must divide 6! by 2.
Since 1 ton is 2,000 lbs, each Crib Jack can life 30,000 lbs. It's going to take 2 Crib Jacks, because the extra 5,000 lbs in belongings were removed, leaving the house at a weight of 60,000 lbs.