Single & multi sensor code
This is the quick and dirty way, I would suggest re writing it to use arrays rather then having to declare all these variables and copy paste pinMode, read,writes if statments a bunch of times.  

 int hallswitch1 = 1;  //Pin for Hall switch
 int hallswitch2 = 2;  //Pin for Hall switch
 int hallswitch3 = 4;  //Pin for Hall switch 
 int hallswitch4 = 5;  //Pin for Hall switch

 int val1 = 0;       //Integer for reading Hall statu  
 int val2 = 0; 
 int val3 = 0;
 int val4 = 0;

 int RELAY1 = 6;    // The socket number on the Arduino that the relay1 will go to.
 int RELAY2 = 7;    // The socket number on the Arduino that the relay1 will go to.
 int RELAY3 = 8;    // The socket number on the Arduino that the relay1 will go to.
 int RELAY4 = 9;    // The socket number on the Arduino that the relay1 will go to.

void setup()
{

  pinMode(hallswitch1, INPUT); 
  pinMode (RELAY1, OUTPUT);

  pinMode(hallswitch2, INPUT);
  pinMode (RELAY2, OUTPUT);

  pinMode(hallswitch3, INPUT);
  pinMode (RELAY3, OUTPUT);

  pinMode(hallswitch4, INPUT);
  pinMode (RELAY4, OUTPUT);  

}

void loop() {

  val1 = digitalRead(hallswitch1);               //Read Hall pin status
  val2 = digitalRead(hallswitch2);               //Read Hall pin status
  val3 = digitalRead(hallswitch3);               //Read Hall pin status
  val4 = digitalRead(hallswitch4);               //Read Hall pin status



  if (val1 == HIGH) {digitalWrite(RELAY1, HIGH);}
  else {digitalWrite(RELAY1, LOW);}

  if (val2 == HIGH) {digitalWrite(RELAY2, HIGH);}
  else {digitalWrite(RELAY2, LOW);}

  if (val3 == HIGH) {digitalWrite(RELAY3, HIGH);}
  else {digitalWrite(RELAY3, LOW);}

  if (val4 == HIGH) {digitalWrite(RELAY4, HIGH);}
  else {digitalWrite(RELAY4, LOW);}

}