I have on rare occasions taken both my children in car without a carseat. In those 4 times I really did have NO other option. I won't deny that I felt awful guilty about it at the time but looking back it was the 'best' option available to me.
We used to take ds home in a taxi when we did our weekly shopping. It was only 3 miles and we didn't have a car.... we just strapped him in with the seatbelt in the back. Taxi drivers didn't have a problem with it.
Could the garage give you a courtesy car and strap the car seat into that? Then you would only have to get a taxi to the garage - alternative get a taxi and go via the garage and get the child seat on the way.
Actually, re-reading your post I'm not sure how far away the garage is, only that the nursery is 4 miles away, so just ignore me if it's not an option.
No chance of a courtesy car and I don't think I can remove my filthy car seat and strap it into a taxi successfully in this cold. Also I've not found a remotely polite taxi driver in this area so I don't think they will be amenable to this plan...
put my ds (2.5) in a cab last weekend, with my parents, he sat in the middle seat with seat beat on. AFAIK that is the 'safest' seat - he thought it was great, but made sure everyone was straped in first!