Why is the power less now
repair manual for series 4000 brigs stratton
Yes, please clarify what you mean by less power. Is the engine surging/hunting? (RPMs don't stay consistent). Since your engine runs (you didn't say no power!), your ignition is ok. You need fuel, air and a spark at the right time to run properly. You do have spark - and because it's really not adjustable on a small engine, that part is fine.
Air and fuel is handled by the carburetor. With today's ethanal fuels, it's likely that your carburetor has one or more jets that are partially/fully blocked. Removing the carburetor and then the jets, doing a thorough cleaning of these jets and the passages in the carburetor should then restore air/fuel ratios. Youtube has lots of great examples of cleaning small engine carburetors.