Thank you guys, but there is a problem with my design ---> "Rule number one is to avoid elbows and bends whenever possible. However, the fact is that many installations require at least one elbow. There are two precautions you can take when installing elbows to achieve optimum airflow.
First, allow a 2-3 foot straight run out of the fan before the first elbow. This allows airflow to be uniform before passing through the first elbow. An installation that has a 90-degree elbow immediately after the fan exhaust port will cause air to flow back into the fan. This will reduce fan performance and increase noise.
Second, use a long radius angle to help ensure optimum airflow and minimum airflow noise. The shortest smooth, inner surface duct with the least number of elbows will provide optimum fan performance."
IMPORTANT:
In order to reduce elbows and optimize fan performance, install the fan with the exhaust port pointed in the direction of the termination point. Be sure to use the duct diameter size specified for the selected fan. Reducing the duct diameter (at any point in the duct run) will create substantial static pressure and reduce the fan's performance by as much as 90%.
Hmm 90% wtf? then If I got my 6" fan and I'm using 6"to4" reducer so I'm loosing 90% of fans performance?
So I think I'll have to move the fan almost to the other side of the box get some straight duct, run it thru the box hook it up to the exhaust hole and then connect the filter straight to the square shower grate with no 90 degree elbow just a 4" pvc connector and have the filter horizontally to get the most out of that fan.