The 4 inch would have to go on the high side or the pond wall it can poke through into the sieve area but it has to be on the pond dividing wall as that is where the high side of the sireve will be. the 4 drain pipe also needs to be fully lower than the overflow/ skimmer intake from the pond if its the same you wont be able to regulate the flow between the main drain and the skimmer action.
and if your only pushing under 5000 gph a sieve the size of three commercially bought sieves i imagine would take a few weeks to clog. Unless you had tons of leaves etc. and even if you do and you make the sieve have like a two inch high side at the bottom of the ski ramp if the sieve clogged and you were not home it would ride up over the edge and continue to run not endangering the fish but the sieve would be bypassed
My biggest worry is the fine control of the level of the water in the pond.. I need to work out if there a easy to connect rainwater harvesting to the pond and maintain the level............. ..........Pond level will always be constant it's the cistern that would fluctuate If the cistern is at the end of the loop the main pump pulls from the cistern and only the cistern water level goes up and down and if you put in a lue float valve at the lowest level the float can turn the water on when it gets too low. Or A small pump can be used like i do. My pond evolved it was not planned to have a cistern at first Read my showcase and the build . https://www.gardenpondforum.com/threads/going-for-it-phase-one-12-000-gallon-pond.22563/.
I have a 3000 gallon cistern that sounds like it would be enough for a year but under the right circumstances it can be depleted in 2 weeks but this year I have only added water once.
It needs a way it can easily be cleaned, but as suggested i could make the who sieve into large basket which I could just take out. I could even look at making a little settle type tank/area at the bottom the ski slope and prehaps rig up some form of flush mechanism. ......................YEAH LIKE A GARDEN HOSE........................................ The bottom drain that feeds into the top could have a diverter that forces the water across the bottom of the sieve........................... I THINK WERE OFF ON THIS THOUGHT see above with the main drain being on the same wall as the pond not across from it it needs to be lower than the overflow as well .... and i would make a open troff so the water from the four in is spread out and over flows the pipe and falls into the sieve
I quite like the design, it's quite experimental, as we know building a pond costs a lot of money what happens if this does not work.
In my original design, I was going to build a space for a settlement tank, the problem was it would hard to get a big enough space, the most I think I could do is around 260 to 300 gals, not really big enough, but I could perhaps replace it with a couple of vortex filters.
In this design what would be my fall back? what could I try if it did not work expected, I 'm really just thinking about how to clean at the heavy crud, I happy the bog should do it's jobif done right. Sorry I'm just typing my thought out as I think about them, trying to work out pros and cons. ....................a pond vac or shop vac to clean out like in the bottom of the sieve
Please let me know if I've got the basic concepts wrong.