Rarely a question that I have to answer.  First teeth usually come in
about 6 months (lower central incisors) followed rather quickly by the
upper centrals, then laterals on the lower and laterals on the upper.

Then first molars lower and upper, then canines, and then second molars
(IIRC).  Usually all done by three years.  "Average" and
"cause for concern" are often months-years apart.

Four teeth at 18 months is a little slow, but it makes a lot of
difference when the first tooth erupted.  If that was at 6 months,
I would be a little concerned.  If they didn't cut the first tooth
until 9-12 months, and the eruption sequence in normal, I wouldn't
be too worried.

If the parents are worried, that's what pediatric dentists are
for!  <duh> (not you, them)

JMO without looking anything up and not being a pediatric dentist,
Steve