You can get an adjustable pin wrench on fleabay for a few quid but don't forget to slacken the top clamp before trying to undo it.
If you are changing fork seal you can do it without removing the top caps! let me know and I'll run you through it.
Just had my bike MOT’d and mentioned to the examiner about removing the caps off the forks. He went and got an adjustable 4mm pin wrench for me to try.
It fitted fine (the pins were 6mm long) so I’m now confident in what to buy.