Gerry,
There's a name for a 1/2" dia, 4" long flute, 1/4" radius ballnose: its called a screamer
I'm just picturing what I think a surfboard looks like and I can't visualize why you'd need a cutter that long. I'd do everything I could to reduce its use to a minimum.
However, I'm looking in the KBC tools catalogue at a Niagara four flute extra long reach ball endmill with a 1/2" diameter, 4" length below shank, 1.25" flute length. This tool is a little bit stiffer than one that is fluted the whole length. If you can hold your depths of cut to 1.25" per pass, it would work.
Another option might be to use a larger diameter cutter with 1/4" radius tip (bullnose form) in order to improve the stiffness of the tool.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)