I am refinishing my first hardwood floor, with several that will follow. I have sanded and stained the 80 year old floor with no major issues. Then came the time to apply the polyurethaning....

I brushed on the poly with a good brush, but I think I was guilty of putting it on too thin because there were streaks in the finish. I then put on a 4th coat this morning where I put it on more heavy, but you can see where I started or stopped with the brush. It does not help that when you stand at the door to the room, there is a window directly across with the afternoon sun highlighting all imperfections....

Suggestions? Should i try thinning the poly? Roll it on, use a pad applicator, or ???

Assistance would be greatly appreciated!!!

I't is typical to find imperfections in the reflection from light. With 4 coats of finish the coating ought to protect the flooring and be smooth. The recognized method to criticize the surface of wood flooring is to look straight down from approximately 6 feet with the light shining down, not at an angle.
A reflection would hi light both sanding mistakes and application results.
Drum marks or scratches not smoothed sufficiently will go away with practice.
Application methods include brush, roller, lambs wool applicator, T-bar and spraying. An important consideration for application is to follow the finish products' recommended application rate. This will help the finish flow so the surface is smooth. No drafts over the surface is important, also working out of direct sunlight.

I am thinking the ‘fast dry’ poly is part of my problem - it is drying before the brush marks have had a chance to settle out...

I use conversion varnish which dries quickly. I put a thick coat down so it has a chance to dry more slowly. Quick dry poly that I have used takes a relatively long time to dry compared to conversion varnish, which dries enough to walk on in an hour or less. A cool floor first thing in the morning and windows closed until about 20 minutes after you finish may help.