A system of control-showing cue bids, also known as Italian Style Cue Bids, by which you initially treat first- and second- round controls as of equal importance.
The purpose is to establish at the lowest possible level that the partnership possesses first- or second- round control in every suit, with a subsequent 4NT enquiry available to confirm that the partnership has sufficient first-round controls.
East’s 4♥ shows first- or second round control of hearts and, just as importantly, denies first- or second- round control of clubs (East would make the cheaper cue bid of 4♣ if holding a club control). This enables the players to stop safely in 4♠.