It's tempting to say "location" here but I think "position" is more exact. The fourth ventricle is not going to change its location in the brain and suddenly move to the frontal lobe or occipital lobe, but its position may be changed as a result of a tumor or hemorrhage or probably a number of other causes.

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Note added at 10 hrs (2015-10-18 02:02:38 GMT)
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When I do a google search for "position of the fourth ventricle" + "cerebral" I get over 32,000 hits. Some examples:

"The recognition of anterior or posterior displacement o f the fourth ventricle on axial computed tomography (CT) has proven to be difficult because the apparent position of this structure is variable ... "

"However, more subtle asymmetry or change in the shape, volume and position of the fourth ventricle can also be the only sign of a more insidious pathology ... "

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Note added at 13 hrs (2015-10-18 04:48:19 GMT)
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Also:

" The position of the fourth ventricle, tuberculum sellae, and torcula relative to an arbitrary fixed point can be established in virtually all cases ... "

" The obvious lateral shift in the position of the fourth ventricle ... appears to confirm the noteworthy finding that the midline has been shifted to the left ... "