My 2 eldest are 15 months apart, and although it seems like a hard task, it's actually not twice the work, they are now 16 and 15, but when younger, had the same bedtime routine, similar activities, school events, similar food, holidays were easier etc, they enjoy similar activities now, like fighting with each other lol! They still do some things together with a variety of friends. However University time is going to be expensive! Now a 4 year gap seems sensible. We also have a 7 year old, which when a baby felt great, but as she's older she gets bored wants to be with or do what the older ones are doing, and days out to the park or farm arn't as much fun as the older two are out with their mates, so theres me her and her dad. We now have to redo the swimming lessons etc and sometimes make an effort to get excited about things like cows so she doesn't miss out. My advice would be as close together as you can.