4U feels head heavier than 3U for same model Armortec?

Probably true. I felt an AT800 4U, it felt simlar to the 3U. Probably different production batch.

 

The balance felt simlar but the weight was definately lighter in feeling.

 

Is that why my AT900T 4U with grip and strings has a BP of 325mm?

 

The BP of the same model will vary from racket to racket. The companies mainly have a range of the BP so as long as the racket falls into that range it is fine.

 

the balance point of each racket may vary, but a reputable racket manufacturer would probably not produce rackets of significant balance point deviation, at least not feelable. if you can feel 2 "supposedly" same model rackets with different balance point, one of them must be off-spec. some manufacturers may shift the BP for 4U to make it slightly more head-heavy than a 3U, as the swing momentum is sacrified for lighter weight racket.

you know the difference between a 3U and a 4U racket, apart from its weight?

for example, say the recomm string tension of a racket is 30lbs, IF a 4U racket were to withstand the SAME string tension and structural stress during play AS a 3U of the same model, it has to be made with a higher tonnage of graphite. hence, you might feel the 4U being slightly stiffer or more solid when you swing it, rather than more head-heavy, but this feeling is very often un-noticeable.

think of it the other way, if a manufacturer produced a 4U racket using the same tonnage of graphite that they use in producing a 3U, they should lower the recomm string tension. in this case, the 4U would not only feel lighter but softer (more flexible) as well.

for the same spec, 4U rackets are "theoretically" more expensive to produce than 3U, but many manufacturers will just sell 3U and 4U at the same price.