That is generally the case but in 100 degree heat in "desert" conditions you need 1-2L an hour so if you are crossing a long desert section with no water sources 8L may not be enough.  It all depends where the next reliable water source is and how fast and far you hike. 

However unless it is for group use you don't really need a filter system that has an 8L capacity.  I would split it up between 1L bottles and 2L bladders/collapsible bottles. 

For more typical trips I agree 4L capacity is adequate and generally it is not necessary to carry more than 2L unless you want to dry camp that night.

The 4L Gravity Works is really intended for group use.  I have one and personally I had bad luck with it.  It clogged up after a couple of uses.  Replacement filters are expensive.  It's also a bit heavy at 11.5oz.  I have also used the Katadyn hiker and had good luck with that...sometimes it is convenient to pump the water...but it is also a bit heavy at 11oz without any storage capacity but it does have a carbon element in its filter which can remove chemical tastes...good if you travel though areas that might have agricultural run off.

I now mostly use a Sawyer Squeeze which can be used is various ways but I use with a 2L CNOC VECTO bladder (~6oz combined) and two 1L Smart Water bottles in gravity mode and encourage everyone to bring their own filter rather than relying on one "group" filtering system.  Generally I carry drops/tables as backup and generally a stove/pot.