According to the service manual here, the slot closest to the CPU is slot 1. That is confirmed in the System Board Component layout picture. And you are correct that slots 3 and 4 are to be used with two RAM modules. And if you look closely at this YouTube video, you will see two sticks of RAM in slots 3 and 4 as well. And this video (a bit shaky at times) shows three sticks in slots 4, 3, and 2 which is the recommended configuration in the service manual for 3 RAM modules.
I have an old Inspiron 570 that I have been using to run a little consumer grade weather station. The weather station finally gave up the ghost and the new one that I got doesn't need to be hooked to a PC. So, I have decided to turn my 570 into a "shop" PC that I can use to play music and look up repair videos or parts on websites. The PC is working well, I installed an Athlon II x4 640 years ago and I just installed a SATA SSD. I have 12gb ( 8gb stick and a 4gb stick) of RAM in it at the moment from my Inspiron 3650 and it is running fine (upgraded the 3650 to 16gb). I have the RAM in the 2 closest dimms to the CPU. According to the manual those are Dimm 1 and Dimm 2, and the manual says that if 2 sticks are to be installed, that dimm 3 and dimm 4 should be used. In one of the threads about the 570, a Dell moderator said that the Dimms closest to the CPU were 3 and 4???? Was there a typo in the service manual? The reason I ask is because I am gonna be removing the installed RAM and putting in two 8gb sticks (already tried it and they work) and I would like them in the proper slots. Having said that, If the computer boots and CPU-Z says that it is running dual channel, unganged, does it really matter if I am not using the recommended slots for 2 sticks of memory?
new ram in Inspiron 570