It is years since I have used MailMerge, so please be patient.

I am making a “Meet the Team” board, and I have a Word document where I am adding 4 records to a page - 4xA5 cards per A4 page.

Some of the fields for some of the records are blank - I am adding Echoism to explain how to say people’s names. How can I add a Carriage Return if the field is blank, allowing me to ensure that each record is set out identically rather than looking like it is “missing” a line.

Your question raises a couple of issues. The most obvious is that A5 is half the size of A4, so it is difficult to see how you will get four A5 cards to the A4 page.
Normally when a field is blank, and that field is in a line by itself e.g as in an address block, the missing field simply leaves an empty line i.e. the 'missing line' that you describe. Replacing the field with a paragraph break (what you describe as a carriage return) would give you an extra 'missing line'. How are your fields laid out on the 'cards'?
With four records to a page, you should be thinking in terms of a label merge document.

__________________
Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com