Hi Rayn
I think you have to be really careful that you actually want another child and it's not more of an emotional need to pre-empt any regrets because soon the choice will be taken out of your hands - the idea if you don't do it now it will be too late.
You sound like you are enjoying motherhood and all the chaos that a larger family brings, but your eldest has still not hit the teenage stage yet and you would be adding a newborn just as the 'complications' of the teen years start in your older ones. Might be worth thinking about.
Irrational desires don't always have to be acted on and you say yourself on paper it is a bit mad with 4 already and a business to run. If you feel like your are living and working at full capacity now don't have a baby because you feel you may regret it later - it's not necessarily so. The level of regret over not having a fifth child, in my experience, is not the same as someone who regrets not having had any children at all.
Another point to consider is, any unforeseen extra complications like SEN, illnesses or emotional problems, ( I'm not saying this is likely but all children's needs become more complex as they grow) may not be obvious as yet as your children are quite young and an extra child could be the difference between coping or not depending on what life throws at you.
On a more positive note as your children grow up and you gain more freedom you might actually be glad you stopped at 4. Sometimes we just need to have moved past the baby stage to appreciate this.
I always urge caution on these types of threads when someone already has their hands full, as I could never have predicted some of the difficulties I have faced with my 4 as they have got older.
Coping with the under 10s is very different to the teen years and I don't regret not having number 5, ( which I ummed and arrhed about at the time.) Knowing what I know now I honestly think that an extra child would have been the straw that broke the camel's back so to speak. Our eldest was 10 when we were contemplating a fifth and the following year life became so much more difficult for a variety of reasons we never could have predicted. I'm always grateful we didn't rush a decision.
Eight years down the road it was definitely the right decision for us and I have no regrets. You may find you absolutely have to have that fifth baby but whatever your decision is, don't rush it. Talk to parents of older children to get an idea of coping with a large family through various stages before you take the plunge.
Lots of luck, 