Congratulations on you "tray success." Some thoughts which might help with 4x5s (I'm just in the process of developing 50+ negs from a vacation trip, about 7 runs of 8 negs each...). First, tilt the trays towards you by placing some scrap wood at the far ends - that way any negs which get away float towards you, and more importantly, it keeps the liquid at the working end deeper, which minimizes scratches. I would suggest "googling" for Ken Lee's website, which has a bunch of tray processing suggestions. The one I picked up from him is using plastic food storage containers, roughly 6x8, from the supermarket, instead of trays. They are deeper than trays (again, the depth of liquid minimizes the potential for scratching, since it keeps the shuffled neg further from the others when you put it back in the chemicals) and has less surface area (which reduces oxidation on rapidly-oxidizing developers like the PMK I use). And, since I keep stressing depth of liquid, don't be stingy with the amount of developer and fixer. I use 2 liters for each run (PMK is very dilute, so 2 liters is inexpensive, and I re-use the fixer based on the manufacturer's capacity numbers in square inches; the pre-soak and stop bath are both water, so they're free.)