What's the RMS voltage of this asymmetric rectangle function?
Let's accept your peak amplitudes are +/- 4.1 V.
The true RMS of that waveform is exactly 4.1 V, it has the same heating effect as 4.1 V DC.
The mean DC of your signal is 2.05 V. If you remove the DC, you end up with a waveform with peaks of +6.15 V and -2.05 V. The RMS of this signal is 3.55 V.
I wonder if your meter is only measuring the AC component?