cnaranjo

Well, in general, my approach only works for exact lookups; when you have inequalities, there really isn't an alternative short of lots of well-commented if-then else statements

I would suggest you create a class called IB_Ex_Wrapper that looks something like this:

public without sharing class IB_Ex_Wrapper {
  
  //	Constructor
  public IB_Ex_Wrapper () {}

	// inner class	
  public class IB_Ex_Result {
     public String XXX1;
     public String XXX2;
     public String XXX3;
     public String XXX4;
  
  // inner class constructor
  public Ib_Ex_Result(String x1, String x2, String x3, String x4) {
  	this.XXX1 = x1;
  	this.XXX2 = x2;
  	this.XXX3 = x3;
  	this.XXX4 = x4;
  }
  //	alternate constructor, all results set to null
  public IB_Ex_Result() {}
  }
  // result map
  private Map<String,IB_Ex_Result> keyToResultMap = new Map<String,IB_Ex_Result> {
  							'W045' => new IB_Ex_Result('2317','3112','4606','1425'),
  							'W047' => new IB_Ex_Result('5817','3612','4698','1155')
  							// add more rows as needed
  	};
  
  // getResult
  public IB_EX_Result getResult(String key) {
  	if (this.keyToResultMap.containsKey(key)) return this.keyToResultMap.get(key);
  	else return new Ib_Ex_Result();
  }
  
}

 then your trigger gets simpler and looks something like this:

trigger NewIen on IB_Ex__c (before insert) {
  List <IB_Exs__c> newEx = new List <IB_Ex__c> ();
    
      for(IB_Ex__c o: Trigger.new) {
          IB_Ex_Wrapper ibexW = new IB_Ex_Wrapper();
          IB_Ex_Wrapper.IB_Ex_Result ibexRes = ibexW.getResult(o.External_Reference_Number_Flow__c);
          o.xxx1__c = ibexRes.xxx1;
          o.xxx2__c = ibexRes.xxx2;
          o.xxx3__c = ibexRes.xxx3;
          o.xxx4__c = ibexRes.xxx4;
                  
          IB_Ex__c v = new IB_Ex__c (id = o.id);
              
               newEx.add(o);
            }
        }

 even this could be simplified by moving the assignments into the Wrapper class

Bottom, line, if you have lots of complex conditionals, create a wrapper class, move the conditionals into methods in the wrapper class and keep the trigger simple.  This has several advantages:

1. You can write testmethods on your conditionals without having to do any database inserts and trigger invocations

2. You can hide complexity of logic into well-encapsulated methods

3. Trigger stays easy to read and focuses on what should be limited to trigger logic -- processing the trigger lists, delegating work to wrapper classes

I can't keep going on this one as your business logic is obviously complex but I hope these techniques will help you organize your code to be maintainable and readable