Hi Claire, congratulations on the new baby!
A friend of the family was a few stone overweight when she fell pregnant, she was reasonably tall if that makes a difference, at about 5'8. She definitely developed a pregnancy belly, but not until about the end of the sixth month. Before that she didn't look much different. Apparently she gained very little pregnancy weight and put it down to being super-conscious about what she put in her body, rather than worrying about how much.
I'm sure you'll develop an obvious bump this time around, four stone is a huge amount of weight to lose, well done!!