Is SRID 4326 Lon/Lat or Lat/Lon?

In principle, it should always be lat/lon as that is what the current EPSG database defines it as. Unfortunately, over the years computer scientists have visited and made a decision to use lon/lat as that works for their high school maths mapping to X,Y and is easy.

So whenever you receive a file of coordinates in EPSG:4326 you need to check who sent them to you, if either of the columns exceeds 90 or plot them on a map and see if they are in the right place. If you are using someone else's code you should look inside the code and see what they are doing. Ideally you will see something like:

double ulLon, ulLat;
// Let's get upper-left corner coords
CRS.AxisOrder aorder = CRS.getAxisOrder(reqExtentInTileCrs.getCoordinateReferenceSystem());
switch (aorder) {
    case EAST_NORTH:
        ulLon = reqExtentInTileCrs.getMinX();
        ulLat = reqExtentInTileCrs.getMaxY();
        break;
    case NORTH_EAST:
        if (LOGGER.isLoggable(Level.FINE)) LOGGER.log(Level.FINE, "Inverted tile coords!");
        ulLon = reqExtentInTileCrs.getMinY();
        ulLat = reqExtentInTileCrs.getMaxX();
        break;
    default:
        LOGGER.log(Level.WARNING, "unexpected axis order " + aorder);
        return ret;
}