I think there are pros and cons to any age gap between siblings.
I have a 2 and a half year gap between my first two, and a 4 year gap between the next two. In the first example, I was just starting to take my eldest to playgroup when my second was born..I found it easy to do things with both children because as they grew up they could participate in the same activities..although I did find that my eldest regressed a little ~ she changed from being very independent and walking miles every day into wanting to be carried or put in the pram! she was a bit young to realise the changes...
With the next two, the older one was starting school. She didn't regress so much, but did do some naughty things to get attention (she cut all her hair off while I was feeding her baby sister once!) I did find it easier to deal with getting her ready for school as she did a lot of it herself ~ and she also enjoyed helping with the baby and keeping her amused without me worrying she was being too rough.
What I did find hard was the activity aspect...4 years is a big gap when you want them to do things together and I found myself having to do things on my own with the older one (or I took it it turns with Mr Pippa) as I seemed to be constantly telling the older one we couldn't do this or that 'because of the baby'.
As I said, pros & cons..and I wouldn't like to say the 'ideal' age gap as there isn't one. Different challenges arise for different gaps, so you can only decide on when you are ready and what is best for your family. I will say that the wedding can be rearranged and accomodated much easier than the baby, though ;o)
Good luck!