As others have mentioned there isn't a "test" for ADHD, but there is diagnostic criteria. A psychologist will use a number of tools to determine whether or not your child meets those criteria.
As pps have noted, short attention spans, and high levels of activity are NORMAL and TYPICAL in 4 year olds. The question is whether your child's traits fall OUTSIDE of that normal range of behaviors, and whether those atypical behaviors are a hindrance to his/her functioning.
My son was diagnosed at about 4 1/2 after he was asked to leave his school where he was academically the highest kid in his advanced class, but could NOT, even when he wanted to, be still or quiet for ANY portion of the class. Even his best little friend, who he'd known all his life would say "I like you, DS, but you make me so tired."
If you're concerned, start with the pediatrician. Ours (as it turned out) had been noting signs of "possibly hyperactivity" since he was about 2. If they see what you see, they'll refer you to the next step (probably a psychologist). The process will probably involve the psych doing some observation of your child, and an interview with you and the child. Then if it is warranted, they'll send home some behavior rating scales to be filled out by you and teachers or other people who know your child. Then they evaluate your responses against normal or typical children your child's age and make a diagnosis. Then you think about what treatment plans you're willing to try.
Sorry this is so long.