I have a lot of incomplete torrents I hope to finish most 90-99%.  Around 4000 in the "downloading" folder. Is that too many, does that slow down the system?

Are you downloading 4000 at the same time? 4k is a lot and If you want to download them you have to wait for seeds and don't download them all at once. It will slow down your computer really bad.

Yes, at the same time.  But most are not active.  I'm just hoping someone occasionally seeds to complete them.

You might start then all, see the RAM usage; if slowing down your computer, then you can selectively stop lots.
Set default connection numbers, and as the torrents are out of seeds, you can put 999 into the simultaneous downloads queue.
Do your tests, the client handles easily if your computer got the requested resources.

I'd highly recommend using Queue management for so many torrents.

Settings > Transfers > General > Queuing

Set Download slots to a manageable number then enable Auto-manage Slots for Downloads