If no class 4 FSD's are available, I would sell all weapons, and non-essential equipment to try to lighten your ship so you can get to the next system. You don't have to have shields, discovery scanner, cargo modules, I'd even go to the lowest size fuel tank that will allow you to make the jump to the next system. This will all reduce your mass allowing you to jump further. You have to have thrusters, FSD, Life Support, power plant, distributor, hull plating, etc, but sell off what is not absolutely necessary. If you still don't have enough, the only thing you can do is self destruct and if it doesn't offer you the option of a class 4 FSD on the insurance screen, choose the sidewinder and start over. You'll have all the money you had before self destruct as the sidewinder is free.