sub4

Try basic Trueplay instead of advanced. Some users have found basic tuning makes the Sub more usable.

If Trueplay is getting things wrong, turn it off and work with the manual settings. Might start with the Phase and see if that helps.

Room placement can matter too, grab an extension cord (to avoid the reboot wait) and move the Sub around the room to see if there is a better spot for it.

I have a Sub 4 on my Arc Ultra that is working quite well so your situation isn't normal.

But without Trueplay it doesn’t sound better, still very weak compared to system without. Different placement is making it sound a little better but still not what I expected from sub. Only setting the sub volume very high is close to bass from arc and era 300. Doesn’t make any sense 

Your Sub 4 shouldn’t perform worse than your Arc Ultra and Era 300s alone. Try basic Trueplay, adjust the phase, and experiment with placement. If it’s still weak, submit a diagnostic and contact Sonos suppor your unit might be faulty.

Bass is very dependent on SUB and listener placement in the room. This is how the physics work.

Play a very thumping bass track and walk the room very close to the walls carrying a stack of Post-Its and a magic marker. As you walk the room you’ll notice bass peaks. Mark each “peak” with a Post-It. At a few spots bass will be even stronger. Mark these spots with !!. These peaks will occur more or less regularly around the room. Placing a listener or SUB at a peak location will result in the most bass. About midway between peaks you’ll notice a “null” where bass will be minimized. After some experience with this technique you’ll be able to locate smaller peaks and nulls in three dimensions throughout the room.

In my college apartment there was a very aggressive bass peak. I placed a chair on that spot. It was the favorite spot for male listeners. Bass seemed to explode from within your body. If I moved the chair a few inches the effect was gone.

First try turning off all your SONOS, unplug it from the power supply. Wait 5min and then turn on your Ultra, let it start up completely, then turn on your Sub. (Also remember to have Loudness turned on)

When your Sub is connected to the Ultra, listen to see if it has changed anything, you can then turn on your Era300.

Where does your sound source come from, TV-Airplay-Apple TV?
My Sub4 is set to 0 and has plenty of bass.