There are exactly 9 different solutions for N=10 units with no rod longer than 10 units. Can you find them? With one of these solutions you can measure up to N=13. Can you do better if you can choose 3 longer measuring rods?
If you double the lengths of all three rods in one of these solution sets you can measure even lengths from 2 to 2N units. Introducing a one unit rod to make up a set of four measuring rods you can now measure all lengths from 1 to (2N+1) units. Can you do better than 27 units with 4 rods?