Run-off Results to high?

Are you flushing with water or nuets? If water there is something in those pots you have to keep at IF these reading are correct. Runoff should be within a few hundred PPM in to out. Tap Water would be oh 100ppm-ish so your run off will be very low flushing with water. THe coco itself will give you 100ppm, so a water only flush would finish when the ppm hit about 200-ish. If you are using nuets to flush, actually that is a better way to do it unless there are problems, 900ppm going in having 1300 out would not be out of line. But 4900 is yeah thats really dangerous IF the reading is correct, they do get a little odd at high readings still it was high. 900 in 900 out is pretty much flushed. You should be ok BUT How did it get so high? Do you go to runoff each time?