I thought I had this issue with mine when I first got it. It locked a handful of times from new going into 4th making very calm changes (I’m always gentle through gears if not going flat out) but releasing the clutch and reapplying was all it took to engage 4th. In my case at least this confirms it to be synchro related as “matching the gears” is all it takes. I’ve stuck with it as in 1200 miles it’s probably happened maybe 6 times but more recently I’ve noticed that instead of locking there’s just a very minor abrasive feel sometimes when moving into 4th. Not a crunch, just some feedback through the gearstick you can feel. Again this only happens a handful of times.
It feels to me that the gear feel and change has improved with use so I think it’s simply down to new components needing some time to bed in.
My issue probably isn’t the same one as I know some people were reporting much more obvious problems, having new gearboxes etc but I’m only mentioning this because if as a new owner you get this once or twice in a brand new car, don’t immediately think you have a lemon. If after a year it’s still doing it then yes the alarm bells would start ringing but at least give the car time to wear everything in a bit!