What is the next excellent year for a interstellar mission (beyond Neptune). The main acceleration for those missions are gravity-assists with Jupiter, Saturn, Uranus and Neptune.
Voyager 2 managed the gravity-assist for each of the 4 gas-giants, Voyager 1 used only Jupiter and Saturn. New Horizons, even through it started at a significantly higher speed, will never reach Voyager 2 because it only visited Jupiter (see the related question here).
I wonder what is the next year with an ideal constellation of (at least) the four gas giants to speed up a space-craft, for reaching interstellar space faster?
A safe upper limit should be ($OP$=Orbital Period in years) $$t =OP_{\text{Jupiter}} \times OP_{\text{Saturn}} \times OP_{\text{Uranus}} \times OP_{\text{Neptune}} ~= 4.8 \text{ million years}$$. That's the maximum time needed for reaching the exact same configuration. But of course space-crafts are flexible, thus the least common multiple should be much smaller.
I'm interested in the next good year (and also on how to calculate that).
Added later: Part of the question has already been answered here, and also on Wikipedia on gravity assist. I would like to understand how to calculate the year (teach a man how to fish...), and what the years for three-planet line-up is.