Pretend you haven't heard the bidding at the other table!
You are at favourable vulnerability.
LHO opens 1D (in a 5CM, strong NT, 4+ diamonds style)
Partner overcalls 3H
RHO bids 4H (slam try in diamonds, does not promise a heart control)
What do you bid?
- if you pass, LHO will bid 5♦ which is encouraging (4NT would be regressive) and RHO will bid 6♦
- if you bid 4S or 5H, LHO will make a forcing pass and RHO will bid 6♦
If you do something else, I'll tell you what happens next
What do you do over 6♦? I assume this may depend on what you did over 4♥...

Help

1♦ 3♥ 4♥ ?
P P 6♦ ?