Class 4 weapons...

Personally I see Class 4, 5, 6 weapons as being the equivalent of giant
siege
engines in the middle ages.  Once you had the enemy pinned down and
behind
some good defenses, you brought up the heavy stuff to knock down the
walls.
The disadvantage was that you were vulnerable to sorties from behind the
walls
or relief from the siege from an external source. Keeping these in mind
the 
spaceship application is a giant "siege monitor" with maybe a class 6 RG
or
beam weapon that you park outside the range of normal weapons (beam, RG
or 
missile) and start blowing away planetary defenses such as satellites or
stations.
You are still vulnerable to such things as fighters, forlorn hope
suicide escorts etc.
but the point is you wouldn't bring one of these into the system until
it was pretty
safe.

A scenario could involve a defending planet sending out such a forlorn
hope fleet 
composed of escorts and fighters trying to destroy the monster ship
before it got
into range of the planet.

just some thoughts...

__Binhan