40k 6th: The Pile In Problem

Ok, Here's the situation. 3 Tyranids all armed with Lash Whips, Get assaulted by 6 Bikers.

Here is the starting postion. 3 bikers in base contact,3 bikers within 2 inches,

Here's a normal pile in, the 3 not in base bikers move to base contact. Do they do it at I4 or I1.  Do all the bikers attack at I1?

I will start doing assaults this way then. The 3 not in base bikers cannot make it into base with the Tyranids now, so they will now get to strike at I4/