For someone who wears a suit four days a week, I think I'd want more than four three-season suits before branching out into summer suits. Five might be good, because then you can have one out of the rotation (at the cleaners, etc.) and still be able to let your suits rest for a week between wearings. Six is probably better.
With spare trousers, a rotation of four suits would last a fortnight. If suits do need to get dry cleaned, do two at a time and that still leaves a week's rotation (2 jackets, 4 trousers).
Work trousers take a 'battering' compared to the jacket, even if one keeps their jacket on. I would never get a work suit without spare trousers, the only except possibly being summer suits (which are not really needed in the UK anyway except for a few months at best)