write timer without delay()

I wrote this code to send an SMS if an input is HIGH as you can see in it,but the problem is I have 4 inputs and delay()s are fatal and very wrong if I need to do more than one single thing at a time (I use 4 inputs).

So I need to change delay() with millis() or something else in void loop() ,Send_SMS() and initia().

can someone help me,and thank you in advance.

const int DI = 2;
const int DT = 3;
const int DGP1 = 4;
const int DGP2 = 5;

int value1_old = 0;
int value2_old = 0;
int value3_old = 0;
int value4_old = 0;

unsigned long previousMillis = 0;
unsigned long interval=100;

#include<SoftwareSerial.h>
SoftwareSerial SIM900 (7, 8);

void setup() {
 pinMode(DI, INPUT);
 pinMode(DT, INPUT);
 pinMode(DGP1, INPUT);
 pinMode(DGP2, INPUT);

 SIM900.begin(19200);
 SIM900power();
 delay(20000);  

}

void SIM900power(){
  digitalWrite(9 ,HIGH);
  delay(1000);
  digitalWrite(9 ,LOW);
  delay(5000);
}

void initia(){
  SIM900.print("AT+CMGF=1\r");
  delay(100);
  SIM900.println("AT + CMGS = \"+212xxxxxxx\"");
  delay(100);  
}

void Send_SMS(){
  SIM900.println((char)26);
  delay(100);
  SIM900.println();
  delay(100);
  SIM900power();
}

void loop() {
  int value1 = digitalRead (DI);
  int value2 = digitalRead (DT);
  int value3 = digitalRead (DGP1);
  int value4 = digitalRead (DGP2);

  if (value2 != value2_old && value2 == HIGH) {
    initia();
    SIM900.println("Station 85: Defaut electrique");
    delay(100);  
    Send_SMS();
    value2_old = value2;
  }

  if (value3 != value3_old && value3 == HIGH)  
   {
   initia();
   SIM900.println("Station 85: DefautGP1");
   delay(100);
   Send_SMS();
   value3_old = value3;
 }

if (value4 != value4_old && value4 == HIGH)  
{
  initia();
  SIM900.println("Station 85:DD>1000");    
  delay(100);
  Send_SMS();
  value4_old = value4;
 }
value2_old = value2;
value3_old = value3;
value4_old = value4;

}