The most important thing to remember as a parent in this situation is simply that he is four years old and has the mind of a four year old.  It is likely that he simply DID forget when you asked him to clean up his toys, even if you toldhim multiple times.  The brain of a four year old does not work the same as an adult brain.  Their frontal lobe, which is the part that is responsible for problem solving and reasoning, is very under developed.  Patience from a parent for this reason is crucial.  It is frustrating and it will drive you nuts, but he is not intentionally ignoring you or disrespecting you.  He simply cannot attend or focus on tasks for extended periods of time.  Children's memories don't even really develop until they are 5.  Give him time to grow and develop.  If he continues to be inattentive and easily distractable, take him to be assessed by a child psychologist.  It may also be that he suffers from ADD or ADHD, which there is treatment for.

It will be important to try and keep your cool with him in the future.  Your thoughts about spanking and yelling are spot on.  Ruling by fear is not healthy and here is a great article on spanking that I encourage you to check out.  

http://health.yahoo.net/news/s/nm/spanking-kids-can-cause-long-term-harm-canada-study

Please take care and the next time he isn't doing as he is told, take a deep breath, tell him again, and even help him start the task so that he may more easily finish it.

Please take care.

Counselor, Dominic
Boys Town National Hotline-A resource for parents and teens
1-800-448-3000
www.parenting.org