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What type of properties are the one that has been sold and the three that have been bought? Was the property that has been sold ever your client's main home or a furnished holiday let? Are any of the newly acquired properties to be let as furnished holiday lets?
If the house that has been sold has only ever been a buy to let property and the three new houses are also buy to let properties, then you cannot rollover the gain and defer CGT so all of the gain is taxable subject to the annual CGT exemption. The first £11,000 of net gains made in 2015/16 will be tax free and the first £11,100 of net gains made in 2016/17 will be tax free.
I hope this helps but let me know if you have any further questions.