I'd use Garmin Basecamp.

It's free.

I'd recommend Garmin Basecamp, but there is a bit of a learning curve depending on what you want to do with it.

Text editor:
Copy and paste everything between and including the <trk> and </trk> tags from each file into one, so you have

<gpx ...stuff>

<trk>
stuff from track 1
</trk>

<trk>
stuff from track 2
</trk>

etc.

</gpx>

30 second job

Memory map ? likely learning curve as basecamp

Could try FatMap online if you have a Strava premium subs, or just strava potentially?

as per northwind - I've used notepad on windows. right click, "open with..." Once you see it written out, you'll see it's pretty straightforward.

same as the b-quote, italic in/out markers when you're typing in here.

Edit: also as per northwind, I responded to your title, not your post!

I've still done it that way before, in combination with bikehike course creator, take sections of each route - using the "reverse route" and "cut to end" tools to crop the sections you need, then combine them in notepad. Bit faffy maybe, but might be quicker than getting to grips with new software if you're not familiar with it already.

I use https://gpx.studio/ - once you get your head around it it's pretty easy to use

It's straightforward if you are only appending files. If you have multiple overlapping start and end points, want to reverse a section, or only use a bit of each then seeing them all overlaid on a map is pretty much essential.

You can have several tracks open in Ride With GPS / RWGPS, but I'm not sure if that's only in the paid premium version.

Basecamp will have you looking up hitmen who specialise in taking out software developers.

TNW's suggestion actually works pretty well. You might have to hack it open again and re-save it with something slimline though as the properties might not match the element counts. ITN converter is free and pretty resilient when it comes to handling bodged GPXs.

I'd probably just draw up a new route in your preferred platform of choice (Garmin/OS/Komoot) its probably quicker.

OK thanks all.. a few different options to try, was trying GPX Editor, which allowed to have all four gpx routes overlaid, but kept trying to edit bits and making a right pigs ear of it, but will try a few options above..

gpd studio linked above overlays all the gpxs on a map, allows you to trim each gpx if they overlap, and save the resulting route as a fresh single gpx

Thanks Duane, giving that a crack now

There's also Viking GPS software to download (which I use), but like others has a learning curve and might be considered archaic by some but it works for me!

+1 for gpx.studio

You just need to make sure about the direction and order of the sections at the bottom of the screen. (probably makes sense when you try it)