When you say "we have mean value of 4K", I assume you mean "my mean values were taken from a source database with 4K blocks", as opposed to "my table's mean record size is 4K".
The mean value from a tabanalys or dbanalys report is the average record size for the table. It is independent of the database block size. You can use those values to plan the RPBs you will use for the storage areas in your target 8K database.
I take the mean record size in bytes, add 20, divide that sum into the database block size (8192 in this case), and then select the next higher integral power of two as the area RPB.
For example, if your DB block size is 8192 and your table's mean record size is 100, then:
8192 / (100 + 20) = 68.3; next highest power of 2 is 128. One caveat: even if the record size is very small, I don't use an RPB of 256.