Last August, I had a newborn and a 4.5 year old. I didn't get a sit and stand. I like to use a sling or a Bjorn for newborns (so that I have my hands free) and my 4.5 year old would either walk or ride his bike. Sometimes, I pull him in a wagon. We take pretty long walks to various parks around our house and I've not had a problem so far. He just turned 5 and wouldn't dream of going in a stroller now - that is for babies! My 6 month old is out of the Bjorn but I still carry him in a Mai Tai or pop him in the Graco. Until recently I used the car seat with the Graco but we've gotten too tall for the infant car seat so now we are just using the Graco alone. I've taught the 5 year old to hold on to the side of the stroller if we are in a busy environment (airport, mall, etc) - but if he wasn't good at that, I would have him wear a harness and hold onto that as well.
Save the $$$. Make your 4.5 year old the "big sibling / big helper". Mine got diapers for me, sang to the baby, found pacifiers, got burp clothes, wipes, etc. Now my 6 month old is sitting and they actually play together! It is great!
Good luck! C.