Using a water stop?

I use 1+4 for 5 mins on acros 100 and agitate thoroughly for almost the
entire time.  Don't seem to have a problem after rinsing 4 times (first
rinse comes out relatively clear, second rinse comes out purple, third
rinse almost clear, fourth is crystal).

I'm using 1+19 ilford stop.

(I'm still new to developing so take this FWIW)

6 minutes should be more than enough:

<http://www.ilford.com/html/us_english/pdf/rapid_fixer.pdf>

Ilford recommends from 2 to 5 minutes.

   It should be enough. Test the film for residual silver
using the sulfide test.
Kodak Residual Silver Test ST-1
Water                        100.0 ml
Sodium Sulfide, anhydrous      2.0 grams

Place a drop or two of this solution on a clear area of the
film and allow to remain for 2 minutes. Rinse off and blot.
There should be no visible stain. The film should be wet but
blotted when making the test.
 The solution lasts about 6 months in a filled, closed
bottle.
 You can make this test on a clip of unexposed film. Soak
the film in water for about 2 minutes and fix it as usual,
then wash and test. Wetted film fixes at a different rate
than dry film so the wetting step is important.

 To establish fixing time test as above using a clip of
undeveloped but wetted film. Fix in dim light and see how
long it takes to clear. Fix for twice this time. Make a test
with fresh fixer and test it this way periodically, it is a
pretty direct test of the amount of dissolved silver in the
fixer. The old rule for single fixing baths is to discard
them when the clearing time doubles. This is probably too
long for paper but not for film.
 Fuji Acros is a tabular grain film similar to T-Max and
Delta films. These films have an unusual amount of Silver
Iodide in them. Sodium fixer is retarded by the accumulation
of Iodide but Ammonium fixers are much less so. When using
Sodium fixer a two bath system is really necessary. While
the capacity of rapid fixer will be greater using a two bath
system its ability to complete the fixing process despite
the accumulation of silver ions is greater and two baths are
not always necessary. I would not be concerned with a
temporary stain in the used fixing bath.

  The stain on these films is caused by the sensitizing
dyes used in the emulsions. It tends to be persistent. While
it may bind to residual silver halide in the emulsion there
appear to be other factors beside this. The use of a sulfite
wash aid, or even a simple alkaline bath (2% Sodium
carbonate, Sodium Metaborate, or even Borax) will remove it.
If Kodak Hypo Clearing Agent is used follow the wash time
instructions.
  I use 100T-Max and 400T-Max routinely. If treated with
wash aid it comes out perfectly clear.
  BTW, tabular grain films fix out more easily than is
sometimes stated. I tested with the use of a two bath Sodium
Thiosulfate (conventional) fixing system and found that the
Sulfide test shows no indication of residual silver after a
total of around 12 minutes in the fixer (6 min in each
bath). However, I did us wash aid which can make some
residual silver complexes soluble which would otherwise
remain in the emulsion.