Measure exactly 4 litres with only a 3 and a 5

Pointer: tap a jug to select it, then tap another jug to pour into it, or use Fill / Empty on the selected jug.

Keyboard: press a number (1, 2, 3) to select a jug, then another number to pour. F fills, E empties, Z undoes, H gives a hint, R resets, N moves on.

Pouring stops when the source is empty or the target is full, so a pour never spills. Par is the shortest solution, found by a breadth first search when the level loads. Levels without a tap start with a full jug and cannot be refilled.