Is there any sample PLC code or code for sale that would control a
constant ratio of four ingredients being mixed together?  The code
will pull four percentages from a recipe list, and during ramp up,
master rate, ramp down, and hold rate, keep the percentage of
ingredients constant.  PID control has analog outputs to control
valves and pulsed inputs from flowmeters.  Similar code for other PLCs
would be acceptable, but Allen Bradley RSLogix 500 for MicroLogix 1500
PLC is preferred.

first let's talk about I/O nescessary for the process. you need to
first focus on the process. pulsed inputs from flow meters is good,
but constant pressure should be maintained if possible, because as
flow increases pressure will decrease if the flow is "starved" for
volume. a restricted flow at constant pressure is controllable.

I would start by creating a PID function for each leg of the 4
ingredients being metered.

these will maintain the flow of each at the specified level. then the
setpoint must be modified for mixture, and offset for pressure
variation from the upstream process.

do all require a constant flow mixture, or is it mixed in "batches"?
if in batches can the batches be sequenced? 

the pid slew rates should match for equivilent change over time, and i
would recommend ramping the system up with a seperate pid that will
filter the setpoints up to command over time using slew rate as a
filter.