wyvern two legged dragons 

wyvern's theoretically would have evolved wings out of hands, as in these pop culture and academic examples: Thus, only two feet.

Remember to keep the tail long, meaty, and quite stiff-it's used for balance. 

There are many different designs you can base on this anatomy. For example, your "wyvern" can use their wings for walking. Feathers are acceptable as well in this evolutionary line. 

Therapsid dragon

four legged dragons from pop culture and historic sources 

Four-legged dragons are more rare in fantasy; they are often portrayed as a less animalistic than a wyvern - and more regal and noble.

The tails of therapsids are flexible and are used more for communicated than for a balance. Dragging it on the ground is fully allowed. To understand how this kind of dragon moves you can observe movements of big cats.