Before you put too much money in this, check out handicappedpets.com
The do have 4 wheel carts for four or front end support. And they have an excellent reputation.

https://www.handicappedpets.com/adjustable-dog-wheelchairs
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Bonnie

With approval of my friends:

SamCat, 03/2011

Seiko, Solo, Sophie, Skye and Buddy - bridge dogs who taught me so much