I have a raster layer that has cell size(x, y) 0.008928, 0.008928. I download it from the website as 1km resolution data under WGS_84. I can not find the unit for these numbers in the metadata. Is there a proper way to calculate the cell size in meters? I need to resample them to 4km resolution data.
Since the data that you have in WGS 84, this means that the cell size is in degree unit. In order to get the cell size in meter, you need to change the projection of your raster from WGS84 to meter projection such as UTM or any other projections that is meter unit depending of the size of the study area. To change the projection of your raster data in ArcGIS go to
ArcToolbox -> Data Management tools -> Projections and Transformations -> Raster -> Project Raster. The Understanding Map Projections PDF file from ESRI has detialed explanation about map projections.