It could be any number of your choice. Pick a number and I can find you a quartic (power 4) polynomial which will give the above four and your choice as the first five terms.
But the simplest cubic rule is
Un = (n3 - 6n2 + 8)/3 for n = 1, 2, 3, ...
which gives
U5 = 5
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The next term is -2.
what number comes next 1 -8 -16 -24 -33
41 t(n) = (9n4 -110n3 + 477n2 -844n + 480)/12
16' then 25 and then 36 et.seq. The clue is 0,1,4,9,16,25,36, ... = 0^(2) , 1(2) , 2^(2), 3^(2) , 4^(2) , 5^(2) , 6^(2), ... n^(2) ...
16 is the next squared number.