This sounds odd. The flow restrictor should be matched to the rating of the membrane. The 4 to 1 ratio is the average you will get assuming that the pressure is in the right range and everything is working correctly. If you don't get 4 to 1 you don't change the restriction, you start looking for what's not right.
Just to build on that it may never be 4-1 as temp as water pressure change that ratio 4-1
What you have is two separate items to look at how the membrane is functioning By testing TDS in and TDS out. As long as it is remove 96-98% TDS then your membrane if fine.
Then you have production rate which is where the 4-1 ratio reference comes from. You may have a low water pressure and temp and your output is 6-1 So the best you can do is crank up water pressure to 60psi and water temp to 70 will get you as close to optimal performance as you can get. if you are under 50PSI your production will drop rapidly
In the end let the restriction do it jobs and if your ratio if not close to the 4-1 then you can try and increase performance with better water pressure and if that is not enough then try optimal water temp.
But Water temp is a pain. Basically a 25' coil in a bucket with an aquarium heater will get you there. But too hot and you will ruin your membrane.
The other thing you can do is daisy chain two membranes together using the waste water on one to drive the second one in line. this is what I do and given the higher more efficient setup I do not use pressure pump anymore. I also have my RO unit on a controller so I just hit to go and voila it shut off by itself. but mine has a CO2 gas off system which greatly complicates the whole process as you have to gas off CO2 after the membrane before the DI , then pump the RO through the DI
I am wondering though after all this if the membrane is really not just fine and if you just haven't set it up properly?
Your restrictor should be after the membrane on the line that valve is on, that is your waste water line
but here is a diagram that shows you a typical layout so you can follow it remember to orient the membrane in the same position when determining what line is what
https://dl.dropboxusercontent.com/u/59844524/Compact Illustration.pdf