I’m not an expert by any means and have just been learning about pool related pumps for the last month or 2 in preparation for my pool. I’m not trying to be a downer either but I doubt you will get that find of flow from a water feature pump (especially if the flow is split to bubblers and running at the same time).
To get that effect over a 4” spillway would probably take over 100 GPM. Maybe 30 GPM per foot or 120 GPM total?
I guess if you had the water feature pump pulling from the pool, and the pool/spa pump pulling from the pool in Spillway mode, depending on the size of the plumbing and location of the equipment, you could get some pretty strong flow and could work.
I worry about the pressure required to push the water up that high since the spa is 4 feet above the pool. Answers to these questions will help...
Where is your equipment located (left side from the pool as you look at the house or right, hopefully left side as that side appears to be more level with the pool/spa)?
How long is the plumbing run and what’s the elevation change?
What is the diameter of the pipe running to the spa and the water feature return in the spa?
What pumps are you using and can you pull the manual for the flow rates?
By comparison, I needed to get about 150GPM to my spa for the 6 JetArrays I had installed. I had to put them on a separate pump so not to hit the max flow rates of my equipment, IIRC filter (120 GPM), heater (90 GPM), SWCG (105 GPM).
I Installed a 32” channel drain capable of about 300GPM with 3 x 2.5” plumbing lines on the suction side (1 to the pool/spa pump for heater,chlorinator, filtration, and 2 to the JetArrays pump). I then have a IntelliFlo XF pump with all 3” plumbing and valves and 3” return to the spa with about 40’ head. I’ll be able to get 150 GPM with the pump running at about 3,000 rpm.