I have code that outputs data from the four FSR's to a virtual serial port interface on my laptop
Where is it?
but the data I get is only from the four individual FSR'S.
And the problem is? It is up to you to add, multiply, bit-shift, ignore, divide, raise one value to the power of another, etc. to get the 4 individual values into one conglomerate value.
What I need is code that will recognise that the four FSR's are one
They aren't one. So, you are out of luck there.
which in turn means the system is on, but if there are one or more of the FSR's off then the whole system is off.
You need to compare the 4 values to some threshold. If all 4 values are above the threshold, do one thing. If one or more is not, do something else. Engage your brain, for a change.
I then need the code to be able to differentiate when the system is active, between the pressure of the FSR's on the feet whilst sitting and the change in pressure when about to stand.
Then, start writing.