4 liters isn't much. Neutralize the PH to about 9 or so using caustic soda, baking soda should work but it'll make the sludge MUCH more voluminous and take a lot more to neutralize the acid, mix it very well during the neutralization process till you get the 9PH (roughly) 7-10 should be okay. PH tape is easily purchased and cheap. Once you have the PH at around 9, let it sit for about 2-4 hours, then either slowly siphon or pour off the remaining liquid avoiding pouring off the sludge that has formed at the bottom, this needs to be done without agitating the solution. The sludge should be evaporation dried (put it in the sun) the sludge while not ideally can be safely sent to a land fill IE the trash, the remaining solution should be diluted as much as possible and sent down the sewer. I'd recommend mixing 1 cup of precipitated solution in a gallon container with the remainder being water, shake it well, dump it down the drain just before a shower. If you take a shower every day it'll be gone in three months, you'll have left some sludgy metal residue in a landfill somewhere that will require an extreme acid or caustic to cause it to become mobile again and enter the environment as an active pollutant, and the remaining solution you sent down the drain will have been so far diluted that it's only marginally worse than environmental norms. If you wanna go the extra mile, do what the pros do, evaporate the sludge down till it's nearly a completely dry powder, mix it with a bucket of concrete and toss it in a dump. What little metal is left in the sludge is then evenly dispersed in a concrete block which will slowly dissolve over a few hundred years unless it's left in a neutral inactive environment in which case it'll last a few thousand years. So it even dillutes the possible re-entrance of the metals into the environment.
One important thing to know when you're dealing with concentrated solutions of anything is that EVERY SINGLE last one of those compounds came from raw materials provided by the earth, they're just concentrated and processed. Proper intelligent dilution and reintroduction to the environment of said materials produces the least harm, to both us and the environment.
Large scale metal processing applications tend to send the sludge through a further refining operation to recoup the spent materials, but the scale has to be very large to make things like that practical, usually it's just immobilized in concrete as aggregate and tossed.
The neutralization/precipitation process is used by every metal finishing job shop in the known universe =).
As a side note, after neutralization adding a small amount (not sure how much) of ferric chloride will help precipitate the metals out of the solution. You have to maintain PH neutrality after you add the ferric chloride. Basically the iron is heavy it helps to coagulate and weigh down the lighter metals, dissolved aluminum solution is also sometimes used because it provides very active nucleation sites for other metals, combined with ferric chloride to weigh it down and mixed with a polymer (food grade polymers are available at chemical suppliers) you end up with a heavy dense sludge, and the least possible amount of dissolved/active/ionic metals in the solution.
The EPA standards for some heavy metals in job shop effluent is so high that it's actually higher than for spring water so I think they know what they're doing =)
Everyone in this forum that uses an etching solution or concentrated chemicals of any kind should read this post thoroughly. It's the thousands of people willy nilly dumping this stuff down the drain that's the problem, and the above prescribed method of neutralization, precipitation and immobilization is the least any responsible person should go through. And all it takes is another acid, or base (depending on the solution) some PH tape and a little bit of time for the most basic method. Copper in particularly bad for plant life in high concentrations, heavier metals are bad for pretty much everything, and it builds up over time, hence the importance of removing as much as possible and dilluting the rest as much as possible.