The remaining 4 inneredges are already on the bottom layer, where they belong. The first step is to arrange them in the correct positions. At this point, there are three possibilities:
If NO inneredge is in place, then use the sequence below:
Before 
N B N+ B2 
After 
You only need to do this sequence once.
Note: Ignore the diagrams. As long as you keep the original top face on the top side, this sequence will guarantee that at least ONE bottom inneredge will land in place afterwards.
If only ONE inneredge is in place, then rotate the entire puzzle until the fixed inneredge piece appears on the bottom front. The remaining 3 inneredges need to be swapped either clockwise or counterclockwise.
Exchange 
Exchange 

N B+ N+ B2 
N B N+ B2 
You only need to memorize one of the above. For example, if you choose to memorize the "counterclockwise" sequence, then use it twice to swap the 3 edges clockwise. Once all 4 inneredges are arranged in place, get ready for the last step: INVERSION.
There are 3 different inversion schemes:
For each inversion scheme, you must rotate the entire puzzle so that the inverted edges are positioned exactly like the ones in the diagrams, before attempting the sequence of moves!
~~~~ Case #1: Invert two adjacent edges ~~~~ 

N B N+ B 
Result: 

______________________________ 

~~~~ Case #2: Invert two opposite edges ~~~~ 

N B N+ B 
Result: 

______________________________ 

~~~~ Case #3: Invert all four edges ~~~~ 

N B N+ B 
Result: 
As it turns out, the same sequence was used throughout all three cases.
Now that all the bottom edges are solved, the only thing left are the
Middle and Bottom Faces.