I don't think you'll find a 4-way xlr y-adapter available off-the-shelf. You could combine three 2-way adapters to provide a 4-way split, and there are several of those available at B&H Photo Video. Here is one.
However, keep in mind that the preamp will see a load impedance equal to the power amp's input impedance divided by four. That may produce degraded deep bass response if the preamp has a coupling capacitor at its output (such as tube preamps typically have), and/or an uneven frequency response generally, if the preamp's output impedance is higher than a small fraction of that reduced load impedance.
Regards,
-- Al
However, keep in mind that the preamp will see a load impedance equal to the power amp's input impedance divided by four. That may produce degraded deep bass response if the preamp has a coupling capacitor at its output (such as tube preamps typically have), and/or an uneven frequency response generally, if the preamp's output impedance is higher than a small fraction of that reduced load impedance.
Regards,
-- Al