Nacho, I had never tried your booking.com trick so I plugged in the Marriott in Budapest. The Marriott website shows 2 doubles but they only accept 2 as the max guests. So does booking.com. However, at my request the plat line called them and they said they would allow 2 adults and 2 under 18 (my kids aren't little). So if you see a hotel you like that won't accept 4, see if the plat line can help. It doesn't always work though.