Their tracks were dark,
their trail very black,

The bird that floats in the air,
swoops less swiftly than their leader,

Drudgery was it for the fellow,
who taught all four of them their ways,

On their ceaseless visits,
to the vessel of gold.

I agree with your solution.  However, the solution is given with the assumption that the person that browses through the riddle should have a wider imagination that could view the four fine creatures not merely four kinds of animals but fingers.  Not only that, all the factors that are raised in the riddle to link up the imagination that you have in the riddle.

There might be other alternative solutions that we might possibly think of unless our mind runs wild.  For instance, the black smoke that runs out from the top of the olden train.  Or a car that emits black smoke at the time of the journey.  Solutions could be numerous and might not lead to one due to the strong imagination of the person.  From the way that you draft the riddle, I could think of how imaginative you are.  I believe you would not pin-point my answer is entirely wrong if you could have the same imagination to link up the solution that I have given to the riddle that you have raised.

I wish all the people could enjoy the riddle that is raised by Mr. Penny since his riddle certainly opens up our imagination.