Hi, there,
good news - you can get fit and lose weight without having to stop breastfeeding. Excercise doesn't reduce milk supply. It's a simple recipe - the more you feed, the more milk you make. By 4 mnths your body is so efficient at the process that most women find their breasts just feel pretty soft and normal all the time - not full like they might have felt previously - but they are still making milk fine.
Even elite athletes breastfeed. I remember the controversy about netballer Janine Illich breastfeeding her (I think 4mnth old) baby at the Sydney olympics.
On another note, breastfeeding protects against obesity. Breastfeeding allows babies to develop a sense of their appetite. Many of us find it difficult to understand when we have had *enough*. Breastfed babies develop this mechanism naturally. Breastfeeding women also generally find it easier to lose weight and their bodies return more quickly to their pre-pregnant condition.
Warm Regards
Barb