Sonars use sound waves, which can bounce, and can also interfere with each other. If you’re using 4 of the same active sensor at the same time in an enclosed space then it’s not too surprising if you transmit a pulse from one quite soon after another, and then detect the second pulse with the first sensor as a ‘far off reflection’. That would also explain the high confidence, since sound is expected to spread over distance, so if it gets a really strong signal (from right next to it) at the appropriate time for a 30m bounce then it likely considers it as a very solid reflection, so has high confidence that there’s something at that location.
Unfortunately the transmit frequency of the ping sonar can’t be changed, so you’ll either have to shield them from each other, or coordinate when they’re active/enabled in software so that they can’t interfere with each other. If you’re in a space that’s enclosed in multiple directions then shielding is likely not possible because you don’t want to block off the valid signal direction for the sonars.