Urgent Rpm(tachometer)help required!

I have recently bought a rpm(tachometer) for my coure 2004.Its on cng,well if i'm running it on cng or petrol the rpm never goes above 4000.@4000 the engine starts to scream so loud that it's understood its the limit!Trying to take the rpm above 4000 would be shear stupidity.The max limit of the rmp meter is 10000.Letme tell you guys how the wirings are done by the mechanic(*******)Its the 1st time i'm having aftermarket rpm & he took 400 for such a simple installation!The red one goes into a 15A stock fuse connected to the fusebox.I'm sure it's a positive.There are two black wires.One is attached to a bolt connected to the body(chassis) it's obviously (ground) and the other one runs straighy towards the engine and is connected to a thick stock wirings that runs from the alternator all the way to the battery positive.Now i dont want the rpm to reach it's limit.All i want is correct reading.

The coure manual says(It also has a maximum output of 30.0 kw/5500 rpm and maximum Torque of 65N.m/3200 rpm).But this aftermarket rpm goes to the max of 4000.One thing more there's a switch at the back side of the rpm that has options like 4,8,9 for different numbers of cylinders according to the car.Coure is 3 cylinder & unfortunately there is no option of 3 cylinder,could this be the reason?Well i didn't install this rpm for giving my car a nice cosmatic look.What i want is the correct reading out of my engine.Please some one help!As always in chinese products the manual supplied with it doesn't relates to the item it comes with!I'm so pissed off!

Please help me!