This is unfortunately a well-known issue that tuples in Kodkod are represented
as positive 32-bit integer, and so in this case, with a universe of 220 atoms,
220^4 > (2^31 - 1) and so we cannot have an expression of arity 4 in such a model.

(It doesn't matter if that expression happens to be a subset of X->X->X->X
where size of X is small; Kodkod and Alloy4 do not atomize the relations
and so every relation is defined over the universe)

It would not be very tractable anyway, so an alternative would be to
reaify it. For example, if you have a relation of type X->X->X->X,
so you can define a new sig "Quad":
sig Quad { a, b, c, d: X }
Then you can represent a set of (X,X,X,X) tuples using a set of Quad atoms.

(Another benefit of doing it this way is that you now have control
over the scope to set for this Quad sig)