A starship needs a bridge equal to 2% of its hull displacement or 20 tons, which ever is larger.

In other words ships below 1000t need a 20t bridge regarless of their actual size, ships over 1000t use the 2% formula.

The cost of the bridge is 5000Cr per ton of ship, the size of the bridge itself does not matter.

And by the way, for a ship to make 2 jump 4s using fuel in the cargo hold would require 80% of the ship given up to fuel, 40% in regular tankage and a cargo hold that's 40% of the ship.