You can't pass through an enemy unit's zone of control unless you're moving to "base" them during a charge.  Once the enemy unit is based, it loses its zone of control, so attacking units can much more easily go "around" them... you just can't go "over" the models.
In your example, once the first four Eldar units base the four Marine units, the remaining four Eldar units may move as they wish to base the Marines, within their normal movement.
If you had limited movement, you could even do something like this:
Code Sample:
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EMMMME
EEEEÂÂ
You can even position your bases, if they're rectangular, so that the "thin" edges of two attacking bases touch a "wide" edge on the enemy to double up.