As already stated your question doesn't make much sense.. your asking how to do something that has already been done for you but here are some other ways.
Select Layer 3 -> Click the Filter Menu -> Find the Filter Equivalent -> Apply the Filter -> Select Layer 4 -> Click the Filter Menu -> Click the first menu item it will repeat your last settings
Clip the adjustment layer to layer 3 and then clip layer 3 to layer 4 by holding Alt and clicking in between the layers on the line. (The adjustment layer will only affect those layers so you can put some below it)
Select Layers 3 & 4 -> Right Click on one -> Convert to Smart Object -> Apply a filter to that Smart Layer.... You can double click on the smart object to get access to the two layers untouched and edit them directly if needed for whatever reason. If you do edit them directly you need to save the document that is opened by double clicking to alter it in the main document.
If you rasterize the adjustment layer into layer 3 and 4 and keep the adjustment layer it will be applied again.
If you use filters you might want to convert layer 3 and 4 to smart objects so you can change/remove the filter.
Personally I would stick with what you have... and maybe clip the adjustment layer because it is non-destructive applying them directly unnecessarily destroys information.
More on Smart Objects
http://www.youtube.com/watch?v=wnPrZZ1e0Zg (Didn't watch the video but looks like it covers the subject)