Can my husband still fight for custody of his youngest even if he moves 4hours away from the child?

You're really asking a general question that could turn on a lot of different factors.  If you don't have a geographic restriction in your decree, then your husband can move.  The advantage of a move for a higher paying job is that he will improve his ability to provide resources for the care of having both children.  Essentially, the argument to the judge is that he can better provide for both children.  There are a couple of potential set-backs, however.  The first is geography.  He will make more money, but will have to spend more on gas to visit his other child, which could become cost prohibitive (crunch the numbers).  If one parent moves, the courts will usually past the costs of visitation on to that parent.  On a similar note, the second disadvantage is still geography-- but in relation to the court.  The county where the child resides will still have jurisdiction over the custody case and make decisions.... this means that if your husband still wants to fight for custody, he will have to travel 4 hours each time to do so.  The third potential issue is that the children are being further divided.  This may or may not be an issue depending on how close the two are, but it will certainly be a factor for the judge.  If your husband really wants to take this job, then make sure that he does exercise his visitations to insure the oldest is not deprived of access.  To "beef-up" access, consider using modern technology like video cams so they can see and talk to each other on a regular basis... it might cost him a bit for setting up both children with equipment, but it will at least show the court that he does want to foster (not divide) the connection between the two children.  This is just a general overview.  Courts tend to favor parents that are responsible and can demonstrate an ability to take care of their children.  So in that respect, the move may help.  However, you mention that he's been fighting for custody for two years, which implies there may be some other issues out there.  Before you finalize this deal, you may want to lay out everything with a family law attorney who is familiar with the judge you have been dealing with.  Every judge has some built in feelings on how they think parents should be.  A family law attorney that practices in that court should be able to give you a more tailor answer to your situation.