Let me give my brief experience with 4.9 foundation.  I am a beeginner and
bought my first colony (one) this spring.  A three pound package of bees
arrived in late April.  I installed it in a medium box, with starter strips
of 4.9 foundation in the frame top bars.  The starter strips work best if
they are only 10 mm wide.

The bees drew this out into nice comb, with cells measuring 5.1 to 5.2 mm
from wall to parallel wall. There was some drone size comb drawn also, but
only on part of a couple frames.  In a few weeks I put on a second box with
similar starter strips, which they started to draw.

In mid May, with the lower box all drawn, I pulled one of the brood combs
and put in in the upper box.  I repaced it with a full sheet of 4.9
foundation.  The bees drew it beautifully.  I thought the original package
bees might refuse it, but apparently younger, smaller (5.2) bees accepted
it.  Since then I have been adding two frames of 4.9 foundation every 1-2
weeks.  I place them between established drawn brood frames.

The 4.9 foundation is drawn out very nicely, except that I have a warp in
the bottom of the comb.  I think I need to wire the frames to keep the
foundation straight.  (Is that right?)

I split the colony in June, adding a new queen to the split.  The two small
colonies will continue to make brood size cells rather than larger honey
storage cells.  Or so I assumed.  I will combine the colonies for the
winter, unless they really gain strength and stores.

I will remove the 5.2 comb from the hive next spring at the latest, so all
brood is raised on 4.9 comb next year.  5.2 comb could be placed above a
queen separator, to allow the brood to emerge, then harvest the comb.

My conclusions:
1.  Bees will naturally take to 5.1 to 5.2 mm comb.  There is no need to
use 5.4 or bigger.  If you want easier extraction, use drone comb in supers.
2.  Once the colony is established on 5.1-5.2 comb, 4.9 foundation may be
introduced.  Bees raised on 5.4 or larger need to be "retired" from comb
drawing, by waiting 45 days after introducing the package.
3.  I have not yet counted Varroa.  I will open some drone cells (if I can
find any.) :)
4.  Next spring I will put in a plain foundation or starter strip in the
brood chamber and see what cell size the bees draw.

I would invite fellow readers to communicate with their foundation supplier:
1.  Urge your supplier to publish the cell size of their foundation in
their catalog.
2.  Urge your supplier to supply 5.15 cell size foundation.  It may not be
exactly what bees would make naturally, but it is a lot closer than 5.4!
And it can be used to replace any brood foundation.

Kyle Lewis
Wyoming Newbee