Having a child is about extending your existing family and your dd should definitely be considered in that scenario, BUT ultimately you have to do what feels right for you and you have to weigh up all the considerations - your dd is only one!
If you are a younger Mum waiting may be a good option, but if you are a bit older it may take longer to conceive - what if you wait until dd is 6 and it takes a year or two to get pg? The gap becomes even larger that 6 years - is that a problem for you?
What will waiting vs having a baby now mean in terms of your career/your future?
Do you want to get through all the baby toddler stuff and finally have a more regular life with a 6 year old child only to get flung back into the world of nappies, nightfeeding, sleeplessness again, or would you prefer to get past those stages more quickly
Also with your dd being older there will be considerations such as: an age gap that big means that your childrens interests will be quite different. You may find it harder to have family outings that both siblings are interested in.
Your dd may resent her sibling anyway as she gets older if she is made to feel that she has to help "look after" the baby. (She may not, but these things are impossible to predict)
Ultimately you need to discuss it openly with your dp and come to a decision that suits you both. You need to factor how your dd might feel into any decision, BUT it should never be made on those grounds alone imo!
Btw - I am likely to have at least a 4 year gap because we are ttc (ds is 3) and it is taking longer than it did with ds, I don't care what the gap is I just want another child in our family.
I am only 20 mths apart from my sister and whilst obviously it meant that there were times when she got attention and I didn't (and vice versa) it did mean that we grew up understanding from quite an early age that we were not the centre of the universe. It helped us develop certain skills such as sharing, empathy and conflict resolution !