The 4th level of the fst(x) hierarchy is denoted with ### it works in the same way as the second and third level the only difference to it and the second level is that it uses fst[x](n)##(p)as input of fst(x). For example fst[5](8)###(13)is is equal to the fst[4]##8th term of 13 in the fst(x) hierarchy for more details into this see my previous blogs. fst[x](n)####(p) or the 5th level in the hierarchy works in the same way as the 2nd,3rd or 4th level the only difference between it and the 4th level is because it uses fst[x](n)###(p) as the index of fst[x]. For example fst[6](10)####(100) is equal to the fst[5]###10th function or term applied to 100. We can continue the process on and on with the higher levels in hierarchy producing larger and larger values but eventually we would reach a point where it would be impossible to determine how many #'s went into creating these numbers. Too fix that I have devised a symbol Q to represent the 3rd layer of the hierarchy above the #'s.