typically you do not need to tell us anything about your project when your question covers everything we actually need to know.
you want to count RPM and display them ? very easy.
http://playground.arduino.cc/Learning/Tachometer
use an IR or infrared sensor and use some white-out or white paint or other 'thing' that will reflect the IR light.
the IR sensor will send out a light, the white will reflect it back. each revolution, you get one pulse.
you count time of pulses. it is very easy.
it works exactly that way if your machine is a bicycle, a rock crusher, a paper shredder, a lathe or drill press. what your machine does is not important to getting the RPM to display.
if you want to have it run for 'about' 4 days, you can use the internal millis() routine.
push a button, set 'start' to be millis()
then if start is 49 hours, stop.
more about millis() can be found in the example programs and the references.
again, what your machine does for those 4 days in about as important as what color the glass is to a thirsty person.
a note about timing, the main board might loose some minutes per day. in 5 days it should be pretty close to the time. if you want it to the second, then google arduino time and there are ways to get the internal clock spot-on for 5 days.
if you want a much longer time, or guarantee that you are accurate, you can get an real time clock or RTC from e-bay for less than $1,00.
if you are after speed control, then swap out the motor for a salvaged motor and speed controller from a treadmill. often free.
typically a 1/2 to 3/4 hp motor with matching speed control that works off a simple pot.
you can turn a pot with a servo if you want to isolate the signals.
now, the really important questions, what color is the machine... did you weld it or use epoxy ?
I noticed green caps on the grease ports. why did you use green and not red ?
the bottles, are the glass or plastic ? why are the lids blue ?
i that liquid have a phase change over 300 degr ? is it a non-Newtonian fluid ?
please note that I am tired and just have an odd sense of humor.
but, since I am in the USA, do you think if I build one and put my gravy for turkey on it, that it would get the lumps out in 4 days ?