Claim 1: Suppose a partition ##\pi_1## of ##N = \{1,2,...,9\}## has 4 distinct numbers, say ##a_1, a_2, a_3, a_4##, such that ##\pi_1(a_1) = \pi_1(a_2) = \pi_1(a_3) = \pi_1(a_4)##. Then there cannot be another partition ##\pi_2## of ##N## such that ##\pi_1(x) = \pi_1(y)## but ##\pi_2(x) \neq \pi_2(y)## for all distinct ##x, y \in N##. In other words, for any other partition ##\pi_2## of ##N##, we can find distinct ##x, y \in N## such that ##\pi_j(x) = \pi_j(y)## for ##j=1,2##
Proof by contradiction: Suppose there exists a partition ##\pi_2## of the form claimed to not exist above. Then we must have ##\pi_2(a_i) \neq \pi_2(a_j)## for all ##i, j \in \{1,2,3,4\}## with ##i \neq j##. Hence ##\pi_2(a_1), \pi_2(a_2), \pi_2(a_3), \pi_2(a_4)## must all be distinct, i.e. each of ##a_1, a_2, a_3, a_4## must belong to a different part in ##\pi_2## and this implies that ##\pi_2## has at least 4 parts of distinct cardinalities. Thus, the number of numbers in ##\pi_2## must be at least ##1+2+3+4 = 10##, but this contradicts the fact that ##\pi_2## is a partition of ##N## and hence must have only 9 distinct numbers. This proves that any other partition of ##\pi_2## of ##N## must be such that there will exist at least 2 distinct numbers ##x, y \in \{a_1, a_2, a_3, a_4\}## such that ##\pi_2(x) = \pi_2(y)##. And since by definition of ##a_i##'s, ##\pi_1(x) = \pi_1(y)##, we have found ##x,y## such that ##\pi(x) = \pi(y)## in both ##\pi_1## and ##\pi_2##.
Now consider the following cases for ##\pi_1##.
Case 1: ##\pi_1## has at least 1 part with a cardinality of 4 or more (i.e. the part is a subset of ##N## with at least 4 elements) or has at least 2 parts with the same cardinality and these equal-sized parts have 2 or more elements.
In this case, it is easy to see that there are at least 4 distinct numbers in ##N##, say ##a_1, a_2, a_3, a_4##, such that ##\pi_1(a_1) = \pi_1(a_2) = \pi_1(a_3) = \pi_1(a_4)##. We simply take any 4 numbers from the largest cardinality part in ##\pi_1## as that part is stated to have 4 or more elements. This ##\pi_1## clearly satisfies the condition of claim 1.
Case 2: All parts in ##\pi_1## have a cardinality of 3 or less and at most one part in ##\pi_1## can have a cardinality of 2 or 3.
In this case, since at most one part can have a cardinality greater than 1 and the maximum allowed cardinality of a part is 3, ##\pi_1## must have at least 4 parts whose cardinality is 1 (single element parts). This is because the maximum number of elements contained in the parts of size more than one is ##2+3=5## (one part of size 2, one part of size 3) and the remaining ##9-5=4## numbers must be covered by single-element parts. Here again, ##\pi_1## clearly satisfies the condition of claim 1.
Cases (1) and (2) cover all possible ways to form a partition ##\pi_1## of ##N##. Since in both cases ##\pi_1## meets the condition stated in claim 1, it follows that it is not possible to have another partition ##\pi_2## of ##N## such that ##\pi_1(x) \neq \pi_2(y)## for all distinct ##x, y \in N##. In other words, for any other partition ##\pi_2## of ##N##, we can find distinct ##x, y \in N## such that ##\pi(x) = \pi(y)## in both ##\pi_1## and ##\pi_2##. Hence proved.