I had this exact same problem. Four children with a gap of eight years between the first child and the second. The second, third and fourth child were all two years apart. Unable to find a marriage license, I thought perhaps the first child was from a previous marriage of the father and the wife had died and he had remarried or maybe this first child was that of another family member, etc. Years later I discovered a child had been born and died in the middle of that gap and between the census. A three and four year gap was not nearly as improbable. Recent DNA Testing with three cousins was the final proof that this first child was indeed a full sibling of the other three.
