I have a 4 year old too and when I want to run with him I use our bike trailer, which converts to a jogging stroller. (I mainly use it in stroller mode.) We have a Burley, a brand which is pretty expensive new, but we got ours used for about $150. I also feel self-conscious about putting my kiddo into a stroller now that he's big; one of the nice things about the Burley is that it's hard for other people to see in, so it's not like they know you're running with a bigger kid
Our "stroller" holds kids up to 100lbs, I think.