Ducting reducers

[stealthisbest]

Sorry if i missed a post with the answer, i tried a search but didn't get anything specific.
in my 2x4x5 tent I switched from CFL setup to 400w gullwing HPS and heat became a problem. The existing system is a 4 inch 190 cfm fan, 4" carbon filter exhausting to attic. I ordered a cool tube that is 6 inch and got reducers to make it fit. I have 4" ducting from filter to reducer on light and same on the other end. In line fan is on outside of tent pulling air to attic.
The question is am I losing flow because of the reducer, but more importantly is there a potential issue with back flow and creating a fire hazard?
And if I may make a suggestion to other newbies, go with the 6 inch setup from the start. The resulting headache from 4" setup isn't worth the cost savings.

 

[TheFertilizer]

Well I do know that the air flow doesn't change, the pressure just gets bigger. So like imagine your garden hose and plugging your thumb over the end, it comes out the same amount but higher pressure, but it also goes further. So I don't think you really need to worry about the reducers. I'm not really sure what you mean about the back flow making a fire hazard?

 

[Bud Good]

You will lose some flow, probably not a significant amount but it depends on what the CFM of the 6" fan is. More CFM requires a larger cross section, that's why you can't get a 1,000 CFM fan in 4". A smaller cross section means less air flow. No fire hazard because of the reducers.

 

[Specialists]

I don't think the volume of air is great enough to cause an issue...