More question, all these probably more to network than NAS though.
1. You know that 4Mbps is 400KB/s max right? You're basically set the maximum speed already, it'll just utilize all the bandwidth.
2. What's the BT setting? Maximum torrent download, upload, maximum peers per torrent, etc.
3. Can you get proper ping reply from the NAS when you cannot login to its control panel? (ping x.x.x.x)

1. I think this should be fine but No.2.
2. Is this number of simultaneous running torrents? Or Global bandwidth? Can you set to only run 1 torrent only at a time?

Even with 1 torrent of small bandwidth setting, if you run 10 torrents at a time also can max out your bandwidth and possible jam your NAS due to overload, try to run 1 torrent a time to check if it happen again.

I controlled my torrent download from NAS by utilizing QoS at router level. By doing this, my torrent will max out the available bandwidth when the line is idle and gives other user priority when they need it.

#in attachment, 192.168.1.5 is my NAS, it have the lowest traffic priority. Using OpenWRT firmware.

oh... didn't know until I read the last line...
Best to change to a better modem/router, as the Riger may not have enough power to process all the heavy connectivity from your BT client thus hang the whole Internet, but if you experiencing the NAS hang as well, that's different issue again.
You can, try to flash the firmware of that Riger to better utilize the hardware (from TPLink).
https://forum.lowyat.net/topic/3224961/all
Follow the guide at your own risk, just prepare some budget so you can get new one if this (save some money method) fail.

merchant9 Reduce the connections per torrent, too many connection will cause congestion on some weaker router.