Sales prediction of an Item
Based on the information given by you. I'm assuming you have performed multiple linear regression ie multiple features and one response feature to be predicted. 

First, apply PCA on all of your features except the response variable you want to predict. In your case the four features you mentioned. Then transform it into a 2 component matrix using PCA. Once you are done with that plot the new Matrix you formed with the response features as a scatter plot.So effectively a 3D scatter plot. 

When you generate this scatter plot you will be able to visualize much better on which regression you have to use. You can decide for yourself if it is linear or not. Depending on how many outliers you are comfortable with.