A514 AKA "T-1" steel. Only comes in letter denoted grades. Each grade reflects a slightly different chemical composition that is formulated to give proper temper on various thicknesses. Example grade A plate comes in 1" and below, while the 4 1/2" plate posited by the original poster would have to be grades P or Q. PW stress relief is NOT recommended for this low alloy steel. You didn't say what process you are using. I successfully qualified several grades in FCAW using ESAB Dualshield 110. For something in that thickness range (I used 2 1/2" to qualify) about 350 F preheat. If the plate is not well soaked, you will get transverse cracking, especially with larger wire.