The next story mission in DLC Pack 2, Omega Shenron VS Super Saiyan 4 Vegeta, has you facing off against Omega Shenron after he has taken Eis Shenron's dragon ball!
You will first face Omega Shenron with Vegeta SSJ4. When you reduce his health enough Goku will show up in SJJ4 form.
After Goku realizes they are not strong enough to beat this enemy, they fuse into Gojeta SSJ4. Keep fighting Omega Shenron until the end!