I hate to be the one to tell you this but 2-3 hrs is actually normal for an exclusively bf baby of that age. 4 to 5 is
very far apart for a baby that young. It actually took me that amount of time (3 wks) to wake my baby up enough to get on a 3 hr schedule because her sleeps were so long. It could be that she is coming out of "womb mode."
Babies also have a need to suck. If you are sure your lo isn't actively eating for the whole 40 min (listen for swallow) then you can offer a pacificer if you desire. My dd wouldn't ever take one. 20 min is a perfect feed at this age. My dd was a 40 min nurser but it took us awhile to get the whole nursing thing down. As your lo gets older she will get more adept at extracting milk.
Yes, it is possible to have a growth spurt this soon too, unfortunately you can expect one at 6 weeks as well.
I would caution you against supplementing if you plan to continue to breastfeed. Let her nurse as much as she wants because she is trying to stimulate your supply.
I hope this helps you...sorry if it is bad news. If you have anymore ? you should also post in the bf board- you might get more responses there.