Find the gallon load of the washer, some are 15 gallon loads. Your have 4 so four times the load. If it takes 3 minutes to fill, there are 60 gallons divided by 3 for 20 gallon loads. If the tankless coil is going to be kept at 180+F and the temerature necessary for a washload is 120F, that is approximately 1/2 the rise from 50F to 180F, so the tankless is required to give 10 gpm while combined with a mixing valve providing 10 gpm of cold water for the mix. This requires a 500,000 output btuh of boiler for the first hour draw, 700,000 btuh thereafter on continuous draw. The load can be spread by drawing on a 120 gallon tank for the smaller boiler size.