Thanks for the further replies. My PC (a Dell Inspiron) was bought in 2010 and I'm pretty sure that it is MBR.
Now that I've realised that life could get complicated (and discovered new terms such as GPT) I've started googling and have come across what looks like the MS definitive approach to the subject of large drives at
https://support.microsoft.com/en-us/kb/2581408.
At first all seems well with detailed instructions on how to initialize the drive as GPT. However, if you scroll down almost to the bottom you reach a section entitled "Known Issues/Limitations" which makes it quite clear that there could well be problems, presumably especially with an oldish setup such as mine. It then finishes with the rather depressing statement: "Contact your storage controller manufacturer or OEM to determine what downloadable support is available for single-disk capacities that are greater than 2 TB."
So I seem to have the options:
1) Buy the 4TB HDD and keep my fingers crossed.
2) Get in touch with my OEM supplier (Dell) and see what they have to say.
3) Buy a 2TB HDD then buy another one when the first one is filled. This would be a bit more, but not exorbitantly, expensive. There would also be the need for a second power socket and USB port. On the plus side, a drive failure on one drive would not be so serious.
Any further suggestions would be gratefully received.