I have a fixed effect panel regression model using 'plm' function in R along with its 'within' option. It has three dummy variables x1, x2, and x3 representing four categories for my binary variables. So if all x1, x2, and x3 are 0, the forth category is 1 automatically. All variables and data are time varying.
I use these in my fixed effect panel regression using 'plm' command with its 'within' option. It has one more numerical variable x4 which is not binary. However, the regression has no intercept when I run the fixed effect panel regression.
Y = ax1 + bx2 + cx3 + dx4
If I want to find the effect of forth category I need to set all x1, x2, x3 as zeros. If so, what should I set for x4 to find the effect of the fourth category?