4♦ was "fit" showing a raise to 4♥ with a diamond suit. Doesn't have to be very strong, something like xx Qxxx AQ10xx xx would be considered down the middle. Partner will have 5 (or more) diamonds about 80% of the time, but could just have 5 hearts and 4 diamonds.
A bit of context for you: LHO definitely considered bidding something other than 1♠, though only briefly. He could have bid 3♣ showing a constructive hand with the blacks (non-forcing); he could also have make a weak jump in spades. You don't know how fast the 4♠ bid came as it was on the other side of the screen, but there were certainly no long pauses.
What do you bid now?
If your call doesn't end the auction, what is your strategy? (if this call doesn't end the auction, it's always possible the next one doesn't either.)
(if you bid any number of hearts or diamonds, or pass, you do not set up a forcing pass; if you cue bid a black suit or bid 4NT RKCB for hearts, pass will then be forcing.)
There will be a play or defensive problem after we've finished the auction...

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1♥ 1♠ 4♦ 4♠
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