using pressure sensor with 0-10v output for 0-10bar using arduino but pressure value won't cross 4bar. so is a voltage divider required in this case?
If we assume your Arduino Uno is powered from 5v, then what you appear to be asking is 'if my sensor voltage always stays within the range 0 to 4v, will it be OK to put it into and ADC that can accept 0 to 5v input?'

Under normal conditions, the answer is obviously yes.

The experienced designer designs things so that they work under 'normal' conditions, and don't break under possible but 'abnormal' conditions. Will your sensor output ever go above 5v? What about a brief blip when it's first switched on? What about water hammer in the pipes you're measuring?

It would be prudent to check how much current the Uno input can accept without damage when the voltage exceeds the rail, and use at least a series resistor between sensor and Uno to limit the current to less than this under fault conditions.