The database shows file 1, 2, 3, 5,6 and 7 of the UK PAL releases.
File 4 is missing. Is it missing from the database or has it never been released on laserdisc?
File 4 is called Colony on VHS.

If it was released it was released after the 3 and 5.
EE 1147 X-Files: File 3 - Abduction
EE 1148 X-Files: File 5 - 82517

I would guess it wasnt released.

If someone still had an Encore laserdisc catalogue from 1999 we might know the answer. I think Encore went belly up tail end of 1998 beginning of 1999. I know Pioneer released the last PAL laserdisc title around June 1999. Sliding Doors was one of the last releases I think.

Edit: File 4 was released in Hong Kong according to the LDDb database. English with Cantonise subs. Files 8 and 9 were also only released there. Seasons 1 through 7 released in Japan.