A 4-year-old boy speaks with a lisp. Should the parents be concerned
about this, and if so, what should they do about it?

Before I provide some online sources of information, I'd like to make
a personal comment. One of my cousins had a pronounced lisp at age 4.
I remember how adults used to coddle him and tell him how cute the
lisp was; other children, however, were not so kind. My cousin was
mocked, teased, and harassed mercilessly by his peers, and he became
very shy and timid. After my cousin started kindergarten, he received
several months of speech therapy, and he became a much happier child.
I urge anyone whose child lisps to seek a professional evaluation.
Here's a site that can help in finding a speech-language pathologist
in the United States (if your ZIP code produces no matches, try
submitting a search with just the state, and omit the ZIP code):

American Speech Language Hearing Association 
http://www.asha.org/proserv/

I've found several excellent online discussions of this subject. This
article was written by a speech-language pathologist:

Preschoolers Today: A Lithp is Just a Lisp! Understanding Lisping in
Children
http://preschoolerstoday.com/resources/articles/lisp.htm

This comes from the website of noted Australian SLP Caroline Bowen:

Caroline Bowen, Ph.D: SLPs talk about Lisping Therapy for children
http://members.tripod.com/Caroline_Bowen/lisptx.htm

Another speech-language pathologist addesses a question specifically
about a 4-year-old:

Help for Kids' Speech: 4 year old with lisp
http://www.helpforkidspeech.org/questions/answer_detail.cfm?ID=356

Here's some helpful advice for correcting a lisp in a child age 3 to
4:

ParentCenter: Can I help correct my child's lisp? Ages 3 to 4
http://www.parentcenter.com/expert/parenting/development/4661.html

The book "Help Me Talk Right: How to Correct a Child's Lisp in 15 Easy
Lessons," by Mirla G. Raz, sounds as if it could be very useful:

Homepages of Mirla G. Raz 
http://www.users.qwest.net/~zoharr/lisp-preview.html

Google search strategy:

Google Web Search: "child" + "lisp"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=child+lisp

If anything in my answer is unclear, or if a link does not function,
please do not hesitate to request clarification.

Best wishes,
pinkfreud

Great - thanks.