I'm a little confused by STOWiki article about those...
The number of Fleet Ship Modules required to purchase a Fleet Ship will vary based on the ship, other ship unlocks on your account, and other factors. For example, if your character already owns the tier 5 C-Store version of a ship, then the respective "Fleet Retrofit" will not cost 20,000 and four modules, but only 20,000 and one module.
If i'll buy fleet ship while our shipyard is at 4, do i need to buy new modules at shipyard-5 to buy a new ship, or the amount of modules will be reduced?
Depends on what you're buying and what C-store ships you already own.
Normal fleet ships cost 200,000 fleet credits.
Fleet Retrofit ships cost 4 or 5 Fleet Modules and 20,000 credits.
However, if you're buying a ship that has a C-store version thats ALSO a Tier 5 ship, like the MVAE or the Excelsior-R, and you already own it from the C-store, then the Fleet version only costs 1 Module and 20,000 credits.
Well some fleet ships requiring only fleet credits others need fleet credits + 1 or 4 fleet modules to get them.
for exemple if you buy Tactical Escort Retrofit from c-store and you to get the fleet one you will only need to pay 20.000 fleet credits and 1 fleet module.
however if you wish to get directly the fleet one for 20.000 fleet credits and 4 fleet modules you won't get the cloaking device.
so if you want to buy a fleet ship that requires fleet modules you should spend some zen for the c-store version of that ship if you want to have the devices too...
this is trick from cryptic to make us spend more money
If we all live in boxes that doesn't mean my box is the same color like yours