[([x] + p) for x in [4, 5, 6] for p in product(*seqs[1:])]
[([x] + p) for x in [4, 5, 6] for p in [[1], [2], [3]]]
[[4, 1], [4, 2], [4, 3], [5, 1], [5, 2], [5, 3], [6, 1], [6, 2], [6, 3]]
Перекрестное произведение наборов с использованием рекурсии