4 BT transmitters talking to 4 receivers is a setup liable to cause delay problems.
If all of the receivers support Bluetooth LE, you should be able to run one transmitter for all receivers, which will improve things. You still have some uncertainty in the amount of delay introduced by each speaker.
Some BT speakers also have a 3.5mm input jack. That's ...
I have been creating and equalizing the speakers from parabold.com and here is my feedback:
low end and high end get truncated through reflection
low end is more present if dish is bigger and thicker
high level frequency response can be equalized by lowering 2kHz-4kHz and boosting 500Hz-1.5kHz (one should be reasonable in the equalization, don't go further ...