I have owned both the 3-wheel and the 4-wheel, and I loved them both for different reasons. The 4-wheel is light weight and folds very small. The 3-wheel is definitely sturdier, and the wheels are bigger.
3-wheel Speed Cart
Pro
Large wheels easy to navigate
Very sturdy
Well built
Cons
Bulky after fold
Fold is a little awkward
4-wheel Push Cart PX4
Pro
Light weight
Sturdy
Easy and compact fold
Cons
Wheels are small and tend to wear faster
Doesn’t free roll as far as the larger 3-wheel
Sun mountain is a US company and stands behind their products, any issues just call the company, return the item and a new one will be sent out. Overall, which ever model fits your needs you can’t go wrong.