Here is my humanoid creature. It is just 4'11 while standing on its legs, but I have not figured out how long the tail should be, other than its longer than the entire rest of its body. Most of the weight is centered in its hips and of course tail, plus a human-sized head. It has a muscular torso and legs if it ever decides to walk normally. The tail's skeleton is built like a snake's body. Would it still be able to move with this tail while keeping itself upright?
Kangaroos can stand on their tails when fighting, so maybe something similar to your creature could be viable. Slithering on its tail could be interesting but it would be redundant due to the creatures legs. If the tail were smaller it could be used to stand on when asserting dominance, or if your creature is sentient it could be integrated into its society as a tool, like a ladder or another appendage. It could walk normally sometimes with its tail behind it and other times jump up on its tail to challenge others or preform tasks. It would theoretically be possible for it to hold itself upright if the human–like part was a bit smaller and it would not try to hold itself up really high.
By strange coincidence, this fact is already well-established. Here is some live-action video demonstrating that a creature almost identical to yours can indeed use its tail to move on land1:
Note that this creature is free to use both its arms and one leg for non-locomotive purposes while simultaneously maintaining forward motion. I believe it is not known whether the creature could move faster if it used all its limbs.
Furthermore, it seems likely that it can walk upright if it can do high-kicks.
1 Well, "on countertop."