MS SQL numeric data type and convertion with removing zero

create table test_int
    (
     num bigint
    )

insert into test_int (num)
values (4096);

My task is to calculate 4096/1024/1024 and get decimal answer. Ok, int doesn't store values after dot, so:

select CAST (num as decimal)/1024/1024 AS decimal, ROUND ((CAST (num as decimal)/1024/1024),4,1) AS numeric  from test_int

First one is pure resault, second one is after rounding:

decimal         numeric
0.00390625000 0.00390000000

The task is to remove empty zeroes after values.

select convert(decimal(25,5), 4096/1024/1024 ,0) 

returns 0.00000.

So how can I get 0.0039 instead of 0.00390000000? Thanks in advance for any kind of help.