why does my Jeep Cherokee diesel take about 4 secs to start\/

....well yes...it's a diesel...Diesel 101 will tell you that you need a couple of moments for the GLOW PLUGS to warm up to combustion temperature before the vehicle will start....I'm guessing that this is your FIRST diesel engine~ there is usually a little green light and you have to wait for the light to go off before you turn the key to start~

In my second career I drove a Kenworth with a 855 cubic inch 400hp Cummins, and sometimes in winter..at like 10ºF it would take 30..45 seconds or more w/o ether. Then it would begin to hit on one or two cylinders (straight 6) for a few more seconds, then run. Summer, 5 seconds cold and at operating temp fired right up. Which is just a tangent story bilko, to compare. It was a 24Volt starter and batteries producing 2000 CCA

Hi if it's a modern diesel in summer temperatures it will start much in the same way a petrol or gas engine will start, as soon as you crank it. If it's caused by glow plug issues, in cool temperatures, then it will fire once the internal compressed air temperature and combustion chamber are warm enough to burn the oil vapour. The engine will smoke badly and rock about far more than normal until all the cylinders are hot enough to ignite the oil vapour individually . Then the motor will run normally until it's stood and gets cold again.