flushing to fast, over a short period (One Night) can shift Ph several decimal points........... which can deliver Ph shock............ is why i suggest flushing to a one or two ltr run off, each night.............. rather than 33ltrs in one flush, with the temperature of the flush water matched to the root temp = important = no root temp shock either......
4.8 i right low, either you have added a substance to the soil when you potted up, or you have been using very high nute stregth over the whole grow period = un used acidic nute build up in the root un used nute....... or this is a right duff batch of grow medium............... probly the latter as you do not use Ec pen to control salt (Nute) input...... we dont know what the Ec was of the Ph4.8 run off, but i bet it was above 3.0 !! = acidic root matrix..... your phos and mag are locked out Boogs
however your new growth at the flowers, is only showing mild tip burn..............
to raise the run off Ph, do this, in 500ml of plain feed water you are using. Dissolve 3-4 heaped tablespoons of "Bi-Carb Of Soda".............. dissolve it, then use it at 30ml inputs to raise your half power nutrient solution Ph to 7.8................ run that in till you get 1 or 2 ltrs run off............... from the last bit of run off, suck up a sample and Ph test it.................. see if the 4.8 moves up to 5.1 or above................. if it dose do the same flush the following night............... see if it raises again.................... the key to bi-carb as a Ph lifter is that it "Accepts" the "Anions" of hydrogen.......bio mops them up......... lowering acidity and raising medium Ph.........................
it's a bit late for a full recovery, as you are near flowers end............... but it may help to load up a few more trichomes than would have been the case if Ph was leaf to sag = drop further towards the flowers end.................. flush with Ph7.0 for 8 days as you have a lot of un used nute in the root matrix that has got to come out before the harvest......... if you dont want the smoke to be harsh on the throat when smoked, get the nutes flushe dout.................... ALR
holy crap, still?! Mate, is your pH meter calibrated recently with 7.0, and better still with 4.0 too? 
... Last bags I checked for pH were in the low 5's! 
.... often found: sticks, stones, bugs reported too! That said, I'm stumped about the pH being this stubborn to correct up.... Hence my wondering about that pH meter,.... 
