I ran your cobination on The Lincoln Electric "Welding Preheat and Interpass Temperature Calculator" and came up with a minimum preheat of 185F on for a 4" thick plate using CMTR we had here for 516 Gr 70 plate. I do think that your preheat is conservative. You are using a very low hydrogen process on a low to medium strength material. The thing you have going against you the most is the thickness, which means high joint restraint and high residual stresses. However, since you are normalizing and tempering the joint, as long as you get it to heat treat without cracking, many of the other ill effects that can be caused by too low of a preheat or too high of an interpass temperature are mitigated. (such as high hardness, or grain coarsening) If you are working to a specific code, I would think that the minimum preheat temperature allowed by the code (225F as mentioned by JWright) would be adequate, and it agrees with the Lincoln calculator fairly well.