Significant difference help

Hello everyone,

I have a quick question regarding which statistical test to use to test for a significant difference.

My set of data shows the monthly output of a product for the years between 2013-2017. A new system was put in place in mid-2017 to make the output process more efficient which has led to increased output.

I have calculated the mean output level (units) for each month 2013-2016 and want to compare this to the actual output in 2017.

Which statistical test can I use to show a significant difference for output in the months of June, July, Aug and Sept 2017 which have a higher output level compared to the 2013-2016 average?
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Kind regards,