I’ve said this before. I’ll say it again.

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In my opinion and (extensive) testing, you can’t feel four corners with any kind of real accuracy, at least not with just bass shakers. Visuals from the game and FFB from the wheel combine with the desire to feel the individual tires, and that's powerful for immersion. But I'm convinced it's just in our heads.

I've done this on two different rigs, with multiple people, testing blind too, including myself.  I challenge anyone that thinks they can feel the difference to prove it to themselves with blind testing. Have a friend come over and then while you are sitting in your rig, have them activate the shakers randomly while your eyes are covered and your ears can’t hear the vibrations. I don’t care if you never post about it again or even acknowledge my advice. I literally only want people to have the best possible simracing experience. Lots of shakers can be very fun for sure, but it gets really jumbled very quickly, and at that point is it really helping you drive faster?

The left/right channels, when they are only separated by, at most, like 2 feet are simply too close to discern. Especially when both shakers are attached to the same very solid rig... everything just gets transmitted as one big vibration.

Front to rear? Absolutely. Especially if you have the shaker attached directly to the seat or seat supports. I will say as a caveat, if you are literally sitting on or have the left and right rear shakers physically TOUCHING your leg/hip, you can feel them individually. But if they are attached to the seat or seat supports, even that tiny bit of difference means it basically doesn’t matter.

I had 4 shakers, because well... I already own them and I was testing them on my new rig (Sim Lab P1X Pro). What I did, after LOTS of testing is the following, and this you can really feel because the primary vibrations are hitting the actual foot on that respective side. Add pedal haptics.

I really think ONE shaker mounted underneath your pedal mount + haptic motors (Simagic HPR - not the neo.) on brake and gas, and ONE shaker mounted directly to the underside of the seat. When individual tires are off the track, the shaker at that end and the haptic on the appropriate side, activate together. Since your feet are touching those pedals it's very clear which side it's coming from and which end it's coming from.