Good morning. Is it save to connect three SP350 panels in series and add the fourth SP350 panel in parallel to them (using the second pair of connector on the solar charging cable that comes with the EP500 which I assume is wired in parallel) ?
You need to wire 2 in series and 2 in parallel to keep the voltage even. Two or three in series will get the voltage high enough to work, but 4 in series makes the voltage too high. Get some combiners and connect the two series pairs in parallel to get the full wattage without going over the voltage limit.