You can dilute it more but then it needs to be in the fixer longer. If you add 25% more water (go from 4 liters to 5 liters), I would add 25% more time to the fix cycle and 50% more time to the wash cycle. If the paper is RC, it is not too bad but fiber paper, the longer it is in the fixer, the more it is absorbed into the paper and the greater chance of archival instability. Be sure to recalculate how much life expectancy you have as 5 liters of diluted fixer will not properly fix the same amount as 5 liters of non-diluted fixer.
If you have a rotary drum, you could use that for fixing the print and it would require less fixer.