Welcome to the forum. You won't like may advice, but you are doing OFATs (One factor at a time manipulation). This is an inefficient and often ineffective method for understanding factor relationships. Primarily:
1. Narrow inference space
2. Inability to estimate interaction effects
3. Unrealistic (you can't possibly hold ALL factors constant and change 1)
4. Sub-optimal (you select optimum settings are conditional (i.e., based on settings of other factors). If you find better settings for other factors, your conclusions about the first factor are moot))
5. Costly (number of runs to optimize is significantly greater than DOE)
I suggest you design a multi-factor experiment instead. You have described a 4 factor experiment. This could be accomplished in as little as 8 treatments (Res IV).
"All models are wrong, some are useful" G.E.P. Box