You are witnessing the loading of your generators. It's one of the things that ruins perpetual motion schemes such as connecting a generator to your car to power the electic motor that's driving it. The electrical load on your generators will put a mechanical load on their rotation and slow them down.
You have another problem too. You are series connecting four alternators that are not synchronised in phase. If they are running slightly different frequencies then they will "beat" as they go in and out of phase with each other.
Figure 1. The sum of two sinewaves of different frequency. The "beat" is the difference between the two frequencies. Sourc: Vibration School.
If your voltages were a bit higher I would recommend rectifying and smoothing each generator but with such low voltages the diode losses will probably be unacceptable.
simulate this circuit – Schematic created using CircuitLab
Figure 2. Conversion of each alternator output to DC solves the phasing problem but voltage drops on the bridge rectifiers will be very significant at low output voltages.