any DOT 3 fluid is fine. i believe DOT 4 is synthetic, or at least the majority of DOT 4 fluids are. dont use synthetic, as it will exacerbate your possible leakage. you just open the cap and fill it up to the middle of the min and max lines.

check under the vehicle after sitting overnight and see if you see any drops of fluid from the corners or under the master cylinder. could be a simple as a slightly loose drain plug on a caliper, a leaky o-ring, or a faulty master cylinder.