Lydia speaks very well - full sentences, almost always clearly and easy to understand. Her vocabularly is large. She has a basic grasp of plurals (one foot, two feet) but sometimes pluralises incorrectly (two sheeps). She can count to 20, can recite the alphabet, can tell you her full name, where she lives (although she's convinced that our house number is 5 instead of 9), and how old she is. Etc. etc.
Of course it's not perfect, she has some grammatical problems (she mixes up I and You), often refers to herself in the third person, sometimes makes up words (if she doesn't know the name for something she'll call it a 'machit thing'), she has a very slight lisp, but other than that she's very fluent.
All children develop at different speeds though, I wouldn't be worried if I were you.