The vast majority of acoustic guitars should use the EP001K. The EP004K is designed for classical guitars, or guitars with fan bracing that have a center brace. The extra 4th piezo on the EP004K will not provide a significant benefit in terms of volume or frequency detection in comparison to the EP001K. A center brace blocks proper placement of the middle piezo element of the EP001K - so the 4-piezo model lets you place one piezo on each size of the center wooden brace. In general, we recommend using the EP001K for any 6-string guitar that does not have a center brace. For more details about piezo placement you can visit this page.