I think it is perfectly natural for yo to want to have a child with your new partner, regardless of how many you both already have. It's all part of our make-up...
If you have the finances & energy to cope with another then go for it! you are both young enough and the eldest kids are old enough to help out too :o)
When I met Mr Pippa he had a daughter (lives with her mum and us at weekends) and I had 2 kids. We have had 2 kids together, and bearing in mind the oldest was 7 when we had our first there was a lot of work involved! we now have a 17 year old, two 14 year olds, a 9 year old & a 6 year old. A lovely family, which I am sure yours will be too..if not already!
Good luck xx