Joining tracks

I had to break up what should be a single mp3 file into 4 separate files while
digoitising. I believe Audacity can join them together into a single file that can be
played. If I do this using the Linus cat command, there can be loud cracks at the
joins, I am told, that can damage a loudspeaker.

So, how do I do this with Audacity?

Never had any loud cracks joining mp3s together.
If they sound okay as a group i suggest leaving them as they are.
As i understand, when encoded, there are a bunch of passive 0 dB samples
tacked onto the start and finish of an mp3 due to the way encoding works so
it should follow that an event such as a loud crack is theoretically impossible.
Joining successive files can be done with File > Import > Audio and then copy/paste the tracks over.

MP3 encoding should be reserved for the final step in an audio production. There is always some sound quality loss when encoding as MP3 and the loss cannot be recovered, Better to use a high quality format (such as WAV or FLAC) throughout the production process and convert to MP3 as the final step if MP3 is required (best to also make a high quality WAV or FLAC backup before that final conversion).

If you import the files into Audacity (File menu > Import > Audio) the tracks will be lined up one below the other, Then with Audacity 2.0.5 you can use "Tracks menu > Align Tracks > Align End to End) to roughly align the tracks end to end. As Jorronno wrote, if the files are in MP3 format then they will each have a little lead-in silence (one of the limitations of MP3 format). Use the Time Shift tool to make fine adjustments to the positions of the tracks. (zoom in/out as required Audacity Manual).
If necessary you can apply short fades to the start/end of each track to avoid the possibility of clicks. Audacity Manual

Thank you for the advice. I’ll try first using the simple Linux cat command
to concatenate the tracks. And I see that I had better update my older
Audacity to V2.
As to mp3, I am always given this advice. My old ears can’t hear the difference
between a wav or flac and mp3, and the latter take up much less space. I
already have about 30 Mb mp3, and it will swell to about 40 by the time I’m
finished digitising, so saving space is of interest.
This is a great Forum! I get quick answers, without the sarcasm that I get on some
sites, for not being an expert on everything.

The problem is not so much that encoding to MP3 causes the sound quality to be terrible, on the contrary a well encoded MP3 file can be almost indistinguishable from the original. The problem is that the damage (loss of quality) is permanent and is cumulative. Each time the audio is encoded to MP3 (or any other lossy format) a bit more sound quality is lost and the sound becomes progressively worse. After a few “generations” the quality can be terrible and there is no way back. This is why lossless formats (such as WAV or FLAC) are highly recommended for backups and all intermediate production files. It’s a bit like making photo-copies of photo-copies, each generation becomes a little worse until it becomes a horrible mess.