Hi there!
I use a Wemos D1 Wifi Uno and four similar water flow sensors YF-B10.
I know I have only two 5V pins but I read somewhere that I can use GPIOs to power more than two 5V devices only if the need per device is less than 40mA. Which is my case.
So my question, does anyone know how to do that?
Which GPIOs shall I use to connect my four sensors?
Thanks for your help! 
The GPIOs on the wemos output 3.3v, not 5. If you need to supply 5v to four sensors, connect them all to one of your 5v pins.