Filters are used to modify light and enhance contrast. In film days, warming and cooling filters were used to change the colour balance to match the type of film you had. Nowadays, this is done in-camera, unless you have several light sources that are not at the same colour temperature, and in that case, you'd use coloured gels to change the light output from the non-balanced light source. You can use polarizers and neutral density filters, but those are usually used with landscape photography. The only filters specifically used in portraiture are "soft" filters, and even there, you can usually get the same effect in post-processing.
Thanks. I've got gels. And a soft filter is what my wife suggested is used on many "model" shoots. Although I do wonder if filters of some sort would be useful with continuous lights. I do use my F4 also. I'll post the question over in the film forum for that use.