4th Gear issue

[Scot Polo]

Looking for some help / advice.
I have a problem with my Polo's 4th gear.
Going up the gears, when you select fourth it will jump out.
If you hold it in 4th for a bit you can get it to stay in gear.
Anyone experienced anything similar? Any ideas to the cause.
I have it booked in later in the week to get checked out but
getting an ominous feeling of a costly bill.

Thanks in advance