David,
I prefer a brush. A 2" may work well for 8x10, you should try a 1" or
3/4" for 4x5 and save some money. Red sable works best. Get a flat,
chisel like, brush. Some length to the bristles will be of benefit.
Stroke the back of your hand with the brush, it should be extremely soft
and flexable. A great substitute for real sable is white sabaline
(sic?) (a nylon, but very soft) which cost much less. For larger
brushes, I only use sabaline.
Jeff