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Rather than use a scrollbar I would find it easier to set up a datavalidation rule on cell F2 where the Data List of allowed values is B2:E2. In this way it doesn't matter if you miss a year. You can even use non-numeric values so it can be converted to provide monthly summaries.

Also I would prefer to use MATCH to find the column and OFFSET( ) to find the value since it is easier for me to follow.