5amp x 16.8v = 84watts (you have to calc this on max cell voltage, not 'nominal' voltage, unless you're going to try and only charge to 3.7v/cell).

Plus, the charger has a ballpark inefficency of 20%, so to deliver that 84watts,charger would need 101watts from the supply.

You don't hav ethat much spare room allowing for the 20% inefficiency, you can charge 108watts. If you don't believe the inefficiency, put a wattmeter like the plane guys use on the chargers input side and see how much more comes in than goes out. I saw some figures this morning that indicates 20% may be low.

You're good for 4S 5000mah at 1C.