Hey Diana K
Thanks for your answer. The middle is 5" and yes the fish tank is 48.
After really looking deep in the Ikeas website I found a put-it-together-how-ever-you-want-it-yourself.. thing so I could go with the 2x4 and still have the outside look nice.
The stand is made out of 2 units. The two inner parts would not have shelves allowing for the 2x4 or the 2nd idea with the extra shelves.
This would be the outside. Also I could really play around where I want the fish tank but for now lets keep it boringly in the middle (red line is the size).
All that you see is about $250... not bad. It is modern looking and if I'm done looking at it i wont feel bad tossing it.
Thanks for all your help guys.