Most important - get everything ready the night before. There will be times when you're exhausted but it will save you precious time in the morning. Line up the bags, get the lunches made, bagged & in the fridge. The night before make sure what you're going to wear in the morning is good to go the same for the kids.
I always showered in the AM - it helped my wake up. The kids would wake up as I was getting my make-up and hair ready so I actually began doing that stuff in the living room so I could keep an eye on them as they watched Arthur or Sesame St. (I used one of those long door mirrors and leaned it against the ottoman or coffee table believe it or not!) I always made their breakfast after my shower and before they work up and had it ready for them - "finger food" that htey could handle themselves - cut up fruit, cheerios, american cheese stacked and cut in tiny pieces, even cut up left over grilled chicken or tiny pieces of meatballs. Omelette or hard boiled egg cut into tiny squares. I've always been big on making sure they have protein in the morning to carry them through.
A word of caution - I never put my clothes on until we were ready to walk out the door - I learned that after many accidents, spills, kid toothpaste or boogers on my shoulder and having to scramble for a new outfit at the last minute. So once everyone was ready to go, dressed, hair & teeth brushed then I'd run into the bedroom and put my work clothes on.
Encourage your 4 yr old to dress huim/herself and cheer him/her on while you dress the little one. Help your 4 yr old select clothes the night before and tell him/her how you know he/she's going to be able to do alot of things for herself in the morning and you are so proud that he/she's growing up in to a big kid. Set the expectation, encourage and praise and you may find that the 4 tr old will do more than you expect.
Also - plan that you'll have a melt-down mornings once in a while. Like us, there will be days when the kids want to stay home and don't want to go to "school". Those will be the tough mornings - there may be tears. Know that ahead of time and try to plan how you'll handle it when it comes. Every working mom has had mornings when we handled that kind of situation really well - and we also have those regrettable days when we're stressed and worry about being late and we handle the situation just horribly. We yell at our precious little kid. The more we try to rush our child, the more they dig in their heels. You'll beat yourself up over those few mornings and wonder why you went back to work. I found that when one of my kids didn't want to go or just wanted to hang out on the couch if I got down to his/her level and listened to their little heart a,d gave hugs and kisses they ended up being fine and off we went. But I confess I didn't always handle it quite that way!
You will find out what works best for you - and you'll get a rythm. Good luck mama!