4 degrees is the norm for most radar installs but I would be more concerned with getting it as high as possible. Proper installation plays a big role in maximizing the performance of marine radar domes.
To perform effectively, mount the radar dome high enough to give it an unfettered, level and full-circle view of the horizon, but not so high as to be adversely affected by the boat’s pitching and rolling. The higher the elevation, the greater the radar range, but too much elevation can hamper close-in performance because the beam will shoot over targets rather than at them. First off, getting the dome above head level is a must because you don't want to bake yourself and your crew with microwave pulses every time you use the radar. (This is less of an issue with Broadband, which emits a much lower burst of power.) Beyond the safety concerns, height is important because the biggest limiting factor relating to radar's performance is the Earth's curvature. Those microwave beams can't be bent to follow that curvature, so the height of your radar antennas and the height of the target you're looking for will always limit the distance your radar can see. A radar mounted 12 feet off the water might first spot a small powerboat, with a maximum height of 9 feet, at a distance of just under 8 nautical miles. No matter how big and powerful your radar may be, no matter how much it cost, you can never expect it to see beyond that. Nor will it commonly see all the way out to this range; this is merely the maximum you can hope for. Target shape and density, atmospheric conditions, and other variables usually will have an effect on range as well. The radar should be installed so that it’s level while the boat is underway. The bow angle is often greater when the boat is running at speed than when it’s at rest. So, if you mount the dome level with the boat at rest, the radar beam will be aimed upward while underway. To find the correct angle, use an inclinometer or phone app to determine the average bow angle at cruising speed for your boat. Use a wedge adapter to tilt the forward portion of the dome downward to compensate for bow rise.