Look at footnote 1 at the bottom of Table 3.1 (AWS D1.1-2008).
Select the low hydrogen filler metal and preheat based on the higher strength base metal.
I assume you are using the weldment in the "as welded condition", i.e., no stress relief, so any filler metal with the suffixes listed in footnote c are not in consideration without being qualified by testing.
Charpy values have to be specified by the engineer and the procedure tested to verify the welding procedure is capable of meeting the requirements specified, i.e., absorbed energy at a specified temperature.
Based on the information provided, I would qualify my WPS using E7018-H4 to verify I could meet the engineer's requirements specifying the conditions for notch toughness. You better check the ASTM material specifications to verify the base metal is capable of meeting the notch toughness requirements as well. Not all carbon steels have notch toughness in the range required for the application.
Best regards - Al