I've heard kids under four shouldn't use playground equipment taller than themselves. Is that true?

It's OK. Many of these "restrictions" placed on equipment are there to protect the manufacturer from lawsuits. However, that is not to say any age child is ok with any piece of the playground. Accompany your child to the park, and play with him as much as possible. That ot only helps them learn proper usage, but is also a fun time with mom and dad. Once they know proper use, they can do more on their own.
No. Professionally designed and installed playground equipment is usually made for 3 age groups: toddlers (6m-2y), pre-schoolers (2y-5y), and school-aged (5y-12y). The equipment should also be accessible for disabled kids, so there can be activities for all kids. So, it is not just the height of the equipment that matters. The website for the consumer product safety commission has info: www.Cpsc.Gov.