The Four Foot Mattress Size is 48 inches wide x 75 inches long. These are fairly standard Four Foot size bed measurements. Four Foot Size Mattress depths can vary from just a few inches to 14" or more. Many times, actual Four Foot mattress dimensions will vary by one or two inches, usually smaller. There is a 48 inch wide mattress that is 72 inches long which is considered an antique size but the 48 inch mattress generally refers to the 75 inch length.