Do multimedia messages sent with iMessage impact data? When I send a pictures from my iPhone (which has a SMS/MMS plan), does it impact data if I'm on a 4G network, and send it through iMessage?

Similarly, if I disable iMessage, then send an MMS over 4G, would it still use some data?
iMessage is pure data usage, so yes it will affect your data.

MMS use depends on your carrier, but most charge a fixed fee per message, so it may be worth finding out the costs for this.

In the UK at least, there are many high data and unlimited data tariffs, so it's far better to replace all SMS and MMS with iMessage (or alternatives such as WhatsApp) where possible.