Shouldn’t be an issue.
You could just explain in a letter why you don’t have any mail for that 4-month period, and see if you can find some mail dated before that time that you can include instead.
Alternatively, you only need to show mail every 4 months anyway, so you may be able to time it so you don’t need to include anything from that missing time period at all.
For example you could show mail from:
Aug/Sep 2016
Jan 2017
May 2017
Sep 2017
Jan 2018
May 2018
And then you wouldn’t even need to mention those missing months because you would meet the requirements anyway.
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