I don't think you're too old to have a fourth. However, I think four is a lot more work than 3 dcs especially when the youngest two are a baby and a toddler, and you've got two older ones that are still only little themselves. You're on the go all the time. (In some ways having say three preschoolers is easier because there is no school run, they don't have to have friends round in the same way they do when they're at school, and they don't need to do any activiites).
If you stuck at three dcs you would have two dcs off at school or nursery,and would only be managing one little one during school hours (if you're not working). Taking your older two to after school activities etc is so much easier with just one little one. Looking after a baby and a toddler whilst trying to manage swimming lessons for the older two isn't much fun!
When one of the older kids has a party to attend it is a lot of work sorting out the other three dcs. The older one for instance would be too young to be dropped off at a party on his own so you all need to get out the car and go into the place holding the party only to return a few minutes later with your ds safely dropped off.
Trying to do homework, with a baby and a toddler needing your attention, for your two older dcs can be a challenge!
It is however wonderful having four dcs but the logical issues and the fact that your older two dcs are really only little themselves does mean things can be a challenge at times. If you delayed having your fourth for a few years you would probably find things easier since you wouldn't have a toddler on your hands as well as a baby, but then unfortunately you would be getting older and the age gap would be bigger which you may not want.
I feel a bit bad being negative about a fourth because my fourth has completed my family and I'm so happy I've got her, but telling you that there isn't much difference between a third and a fourth would be untrue in my opinion; although ages of the children can make a huge difference to someone's experience of having four dcs.
Good luck in whatever you decide to do.