I would recommend against steel. Makes good targets but is lacking as armor. You can get quality ceramic level 4 plates for the same price (Hesco 4400/4401) that are NIJ certified, proven, and lighter. For a budget lighter weight plate that is special threat (m855) look at Hesco L210. You can get those around $350 and they are 5 lbs and some change.
Got a link to theses plates you are talking about. I am interested.