after 3 mor 4 years does the entire surface need to be sanded down to bare wood to add additional coats or can it be cleaned and painted over?

If you are applying another coat of the Semi-Transparent Stain and the current coating has weathered sufficiently to allow new coating to penetrate into the wood then another coat of stain can be applied. Otherwise the previous stain should be either sanded or stripped prior to applying additional Semi-Transparent Stain. If you are painting over the stain then the surface should be clean, dry and sound prior to applying the paint. We would recommend applying it to a test area first prior to applying it to the entire surface to ensure your satisfaction with the performance and appearance of the coating.