Reducer or no reducer when brushing on paint

[The "A" Team]

I know this can be extremely unpopular, but for a number of reasons, I'm planning on applying my new paint job with a brush instead of spraying. I'm looking at a few single stage paints, and going for a semi gloss for a some shine, but not too shiny. My question is, some single stages call for a mixture of 4:1:1 (one part reducer). Do I need the reducer if I'm brushing the paint on?

Thanks.