Since the original poster noted that converting her bike to an longtail wasn't realistic cost-wise, I was going to suggest either the Bobike Junior or the Yepp Junior. Both are Dutch seats made for the 5-10 year old set. But they don't attach to an American rack. It looks like the Bobike Junior attaches directly to your frame whereas the Yepp Junior attaches uses a special rack that has an attachment for the seat. http://www.bobikeusa.com/Bobike-Junior-BB-JR.htm
SF mom who has one (see last photo): http://humofthecity.com/2012/02/10/these-are-the-ways-we-ride-to-school/
- Another site I read suggests this doesn't attach safely to all bikes - you might want to contact http://www.mydutchbike.com/
in Sausalito to see if they have one in stock, in which case you could confirm the fit before you bought it.
Neither seat is cheap (north of $200), but these brands tend to be very durable so your seat would probably have good resale value once you were done with it.
There are also lots of ways to McGyver a seat for cheap. The Hum of the City blog linked above has been featuring some of these lately. To be honest, when I was a lot younger I used to ride my bikeless, 95-lb sister around the Mission directly on my rear rack when I didn't feel like pushing my bike alongside her. I popped several spokes but the bike (steel) is still going strong...and so is my sister.
I think you can use the extracycle rear add on. It sort f replaces your rear tire and tack and i think it has higher weight limits.
But that probably requires a seat as well. I've not used one so can't review it's benefits, etc. http://www.xtracycle.com/
My son is about to hit the weight limit on our Copilot Taxi bike seat. I'm not sure what my next options are given that I don't have the money for an extended bike nor to convert my bike, and a trailer doesn't fit my bike-to-work ambitions (and more to the point, won't fit in the elevator to the bike cage in my building).
Is there a seat or platform of some kind for a forty+ pound four year old that I can attach to my current Blackburn rack so I can keep hauling him around? I'm not even sure which search words to use on Google.