Weirdly, it might be easier to start at order 5 (level 1) than order 4. Here’s an example:

Since two lower nodes sum to the upper node they’re connected to we see that between 7 and 5 must be a six (7 + 6 = 13):

Which means two more nodes are easy to find:

Now working down the right side:

Finally, the bottom row has to be a permutation of: 1 2 3 4 5. The rightmost nodes must be 1 3 or 3 1. Just try something: put 1 in the far right and work the other nodes out. If there’s a problem (there is) then put 3 in the rightmost node.

So start with 3 at the far right and you will see that all is well.

If you don’t get a puzzle with a node that is easy to find, just make a new puzzle. After awhile you will develop a strategy (i.e. a guessing procedure) so you can solve more difficult puzzles.

Lattice order 4, Level 2 look pretty scary, but with a strategy and a bit of persistence they can be solved.

Have fun!
Robert