Hi,
Don't panic! 
Nothing is set in stone yet so take a deep breath.
If you are pregnant and it is an if, you will work it out. You know this deep down I'm sure from when you have had your other babies. Your youngest would be at least 2 and maybe it isn't ideal but its older than a lot of babies are. Plus he/she has been born into a family already with brothers and sisters so I'm guessing its not the same adjustment as if he/she had you all to themselves from the start.
In your shoes I would wait and see. If you are pregnant it might reassure you to look at your finances and budget in the governemnt funded nursery hours and child benefit etc. It might surprise you.
Most importantly I think you do want another baby - it sounds like it is outside influences that are making you doubt.