Yes, you can use four speedlights. I always used to use two flashes for water drops, connected by sync cords.
But if 1/16 power is not enough, then it sounds like the problem is that your distance to flash is too great, and you must not be using ISO. The YN-560 guide number at 1/16 power 35 mm flash zoom (for example) is GN 16 (feet, ISO 100, chart in flash manual). So therefore, for one flash, f/16 is proper exposure ISO 100 at GN16/f16 = 1 foot distance (which works for water drops).
Four flashes would add 2 EV (changes 1 foot to 2 feet).
(2 flashes double to 1 EV, and 4 flashes double again to 2 EV)
Or much easier, ISO 800 using only one flash adds 3 EV.
I have a guide number calculator that can show all of this on my site at https://www.scantips.com/lights/flashbasics1c.html
I used two flashes at 1/64 power and f/16 and greater flash zoom, and exposure was pretty easy. In my case, the flashes were behind a frosted plexiglass sheet behind the splash. The two flashes with color filters were about 2 feet from the splash. Example at https://www.scantips.com/drops/shako/