I was also worried about the age gap, when we unexpectedly were expecting! We had our older kids (3 in 3years). It was crazy, but we liked having them close in age. While there are definite challenges, the older kids are absolutely in love with my toddler! They are so incredibly helpful, I am so grateful! My oldest is even such a nervous nelly with her, that I have to tell him to chill out! :) We took her to the beach and he kept telling me I was going out too far in the water.......or he freaked out when I took her on the jetski with me (slow and with a life jacket). It's kind of funny. Being such a good big brother will not doubt make him a great dad one day.
Our challenges probably stem more from the fact that we have no family or friends we can call up to watch the toddler while we go to a movie, or spend an entire day at a baseball tournament. I feel bad that we can't do these things. I feel bad for my oldest, who's in many sports, that my hubby and I have to "tag team" most of the time - instead of us both being at the activities. The kids say they don't mind, and maybe they really don't. It could be more of that "mommy guilt" that plagues us!!!!
Also, my toddler spends A LOT of time in the vehicle! She's really good about it. (What would I DO without DVD players in the car?!?) She gets schlepped around to so many different activities, lessons, etc. There are days where she wakes up- gets in the suburban. Goes down for a nap, wakes up- gets in the suburban. Her whole day, almost, is driving around.
I am delighted every day I see my oldest with his baby sister. They have a very strong and unique bond. It makes me sad that by the time she's 8, my oldest will be heading off to college. I pray they keep that bond. But, no doubt, she will miss him horribly. But, they will be best buds for now :)
I also joke that my youngest is like an only child.....with bad older influences! The worst of both worlds! LOL! She's spoiled rotten, plus, she will mature faster, having older siblings- could be a recipe for disaster! For example, she can sing most of the lyrics to "The Time", by The Black Eyed Peas!! No "Wheels on the Bus" for her! Yeah, recipe for disaster........ha ha.
Only you can decided on a 4th child, but I just thought I would mention our experience with the age gap. I wish you the best of luck, either way.