It would be a good idea to determine the run order and sort the table in run order to investigate the cyclic pattern observed in the residuals.
The fact that the four factors are involved in different process steps means that the experiment is not randomized. This fact has important implications for the analysis of the data. The statistical errors are correlated. The fundamental assumption of constant variance across the response range is likely violated. The model with a single constant variance is inadequate. The parameter estimates for the effects of each factor require different standard error calculations based on the multiple experimental units (randomization). Otherwise, the type I and type II errors are different than expected, and p-values do not have the expected coverage.
All t-tests in the Parameter Estimates table are (estimated parameter - hypothesized parameter) / (standard error of estimate). The hypothesized parameter value is zero. So the intercept is compared to zero, like the other parameters. The standard errors, however, are unlikely to be correct because of the split-plot 'design' used to collect the data.