You are playing a natural system with 5 card majors and a strong NT. Partner opens 1♠ and rebids 4♠ showing a big hand as 2♠ and 3♠ would both be non-forcing. So you make another move? If so, what?
Note: This is an opponent's problem, so I'm giving you the best description I have of their system.
1♠ - 2♥
4♠ - ?