Is running 46t front chain ring with an 11t on the back such a huge disadvantage for flat cat 4 crits?
Normally, I think most racers have a tendency to be over-geared, but in your case you may find yourself under-geared.  That said, if it is your first crit, just race what you have, the experience is the most important thing. I am also a bit suspect of the win off the front by a first-timer strategy.  If you have the opportunity to win, take it, but don't be disappointed if the race turns out to be much different than you are currently envisioning.  

Anyway, some reasons why you may be under-geared:

Efficiency

When using a chain drive, Big-Small gearing combinations are not as efficient as Big-Big.   You could be putting your drive train losses as high as 5% running such a small gear on the rear cassette.  I suspect you will also be racing a cyclocross bike (which will be a disadvantage), the efficiency loss will be another disadvantage.

Surges

The pacing of a crit is not as constant as a time trial or cyclocross race.   Often there are very fast surges, and your peak speed can be quite fast if you choose to close the gap between groups. I am not sure where you got the peak speed of 58.3 kph (a friend I assume).  Speeds can change from year to year and by rider strategy.  Peak speeds of 60-70 kph are not entirely uncommon, especially if the course has a downhill section or there is a strong tailwind on the day.

A Note on Strategy

Riding off the front on your own in a crit is a pretty tall task.  Doing it at an average speed of 48 kph is pretty monumental.  That would put you into the realm of a top-end professional.

My honest advice is to skip this strategy altogether.  Rather focus on your position with in the peloton, keep moving up, and stay near the front, but don't ride at the front or off the front. Your goal should be not to do anymore work than is absolutely required to keep a good position.  Don't chase down breaks or initiate breaks.  Wait... as painful as that sounds. 

Count your laps, know when you're 10 laps out, then start moving to the front.  Sit in until you are two corners out, then get ready to go, follow anyone moving up. Don't do anything more until you are 400 m out.  Then give everything you got and see where you finish, you might surprise yourself.  90% of sprinting is positioning, timing and reading the race.

Standard Gearing

Standard gearing is 53/39 on the front, simply because the speeds are much faster in a group than on your own.  That said, even in Cat 1/2 I only ever rarely used a 53x11, even in crits where the speeds can often average 55+ kph on faster courses.  A compact gearing (50/34) will likely be fine.