Lets look at what we know and ask if it makes sense.
OK start with 4.4E+6 gallons in 24 hours--how many gallons per second is that?
4.4Eg gallons/24 hrs*1 he/3600 sec = 50 gal/s Good flow but believable.
OK what is the cross sectional of an open 4" gate valve....a gate valve is guillotine across a 4" diameter opening.
A=pi*d^2/4 = 3.14159*4*4/4 in^2 = 12.5 in^2
A gallon is 231 cubic inches--so the velocity of 50 gallons per second divided by the opening area.
V=50 gal/s *231 in^3/gal * 1/12.5 in^2 = 925 in/s * 1 ft/in = 77 ft/s
Now I happen to know that 88 ft/s is 60 mph so this waste water was exiting the open valve at close to 52.5 mph
That's a pretty good clip for close to waste water--it was under a pretty good head (head is another way to express pressure--15 pound per square inches is equivalent to 32 feet of water head.
The beauty of Bernoulli is that his equation id a relation--a relation between kinetic (velocity) and potential (elevation) energy.
OK Potential Energy (PE) is given as PE=m*H*g where m is mass H is height and g is acceleration of gravity (32.2 ft/s^2)
Kinetic Energy (KE) is given as KE=1/2*m*V^2 where m is mass and V is velocity in ft/s.
Divide through by m and g
H=1/2*V^2/g = 1/2*(77 ft/s)^2/32.2 ft/s^2 = 92 ft That a hell of a standpipe.
As 32 feet is 15 psi, 92 ft is 43 psi
That's a pretty good pressure---I'd be suspicious-- this was more than an open 4" check valve, there was a running pump behind it.
Think of it this way--the typical backyard inground swimming pool is 60,000 gallons--4.4 million gallons is 70 swimming pools full---that almost 3 pools per hour.