My first child had a mean extrusive relex.  He didn't start solids until he
was eight months old.  By that time the pediatrician was becoming exasperated
with me...none of his other mothers had trouble feeding their children at the
"right" time.  He eventually sent us to an OT over my supposed inability to
feed my child and Nicholas' inability to take a bottle.  I have to admit I
didn't try very hard  to get him to take a bottle. ( I didn't own a baby
bottle.)  Even at nine months he couldn't figure out how to get the food to
the back of his mouth and swallow, but he had great fun finger-painting with
cereal and creating high chair cement.

When the second baby came along, I thought I had it all figured out.  I'd
wait until she was eight months old and the transition to table foods would
be swift and effortless.  I ignored all her readiness cues.  She tried to
grab food from our plates.  I ignored her.  She'd make chewing motions.  I
ignored her.  She started to pull off fuzz from her sleeper and eat it.  I
ignored her.  She quickly became mobile at four months by learning how to
roll over.  Back to front.  Front to back.  Well, she rolled herself over to
the window, discovered a dead fly and consumed it, wings and all.  She
started solids at four months without my consent.  She loved carpet treats.
 I became a vacuuming fanatic.

Which only goes to prove that babies are all individuals.  I share with
parents how to look for readiness cues, just like the feeding cues in a
newborn.  Can the baby sit up?  Does she have a palmar grasp?  Is her pincer
grasp becoming refined?  Does she want things off your plate?  Is the
extrusive reflex still there?  Does she eat the fuzz off her sleeper?
I think the 4-6 months recommendation is a great recommendation, but the
developmental cues are more important.