Why doesn't 4 yr old speak on complete sentences

Many don't. If your child is still having trouble with sentences, you can get free speech therapy after the age of 3 with most public school districts in the U.S. Try calling the local school or the district office for the information you need.
Of course, they will screen your child first to find out if there are speech delays, etc. When your child enrolls in Kindergarten, the teacher will refer to speech therapy if he/she observes delays. I do this all of the time as a Kindergarten teacher.