int LDR[]={LDR1, LDR2, LDR3, LDR4}; //Assigne value 0 to integer of variable val
So, the array holds 4 values, numbered 0, 1, 2, and 3.
msr=analogRead(LDR[5])*5.00/1023;
You can't read from the pin in the 6th position of a 4 element array.
//switch (LDR[0]){
Switching on a pin number hardly makes sense.
Actually switching on any single value read from any of the LDRs doesn't make sense. A line following robot reads all 4 values, and uses the 4 values to determine where the line is, relative to it's current heading, and adjusts speed to get the line back in the middle.