Well, powered it up just out of curiosity, all I saw was a faint monochrome (B/W) signal, sometimes with stable border and paper with random patterns scrolling vertically, or diagonally, or flashing vertical bars, or still vertical bars. Sometimes, randomly, it could start giving "loading" stripes on the border, with the appropriate sound coming from the internal speaker! Quite a variety, eh?
I noticed that the voltages at the lower RAM ICs were not correct, I only had a good 4.96V in the 5V line, but only 2.30 instead of 12V and -1.35 instead of -5V.
What I have done so far:
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-Replaced 7805 and D19 which were in bad shape, anyway
-Replaced all electrolytic capacitors with fresh ones
-Removed and socketed all lower RAM ICs, also fixed broken traces
-Removed and socketed all the LSXXXXX logic ICs
-Tried all freshly-socketed RAM and logic ICs in a good working Issue 2 board which I have with sockets, they all work (miracle!!!)
-Also swapped all of the above with the ones (also socketed) from the working Issue 2.
-Checked ULA from the 4S on the Issue 2, it works as well.
-Replaced TR4, TR5, D15 and D17 (BA157) with new ones, but also checked the removed components on the working Issue 2, they are good.
-Measured (without desoldering) all other diodes and all resistors around the power circuitry (also the 15 Ohm R61/62 which I read that are common suspects in such a case), they give proper values according to the service manual for Issue 4A and 4B and according to the color codes of the resistors. They all look in good physical shape, too.
N.B. that I was unable to find a specific service manual or schematics for the 4S board, but it seems to be pretty close to the previous 4x boards. The 4S is rather obscure, though and little info about it can be found in the web.
-Replaced the coil with the good working one from the issue 2.
Despite all my efforts mentioned above, each time I fire up (with or without the ICS in sockets) the board and after each separate step of progress, the results are EXACTLY the same as described in the beginning, just the black/white picture quality became slightly brighter and sharper.
The replaced, new electrolytic capacitors C44 and C45 give both almost 2.5V of current, instead of the values they were supposed to give! The capacitors near the power input circuitry (C43, C50 etc) give good voltages, though.
The board gives EXACTLY the same results with lower RAM installed or not, and even with TR4, TR5 and the diodes D15, D17, D19 completely DETACHED.
All I have left to do is desolder the CPU (it seems to reset when I tested it, and all the symptoms described declare that it SHOULD have some kind of activity) and also the ROM and the upper RAM ICs. None of them gets hot during operation. But could it be some sort of short-circuit that draws the current in some way? I am not an expert in the theory of electronic circuits, so I am kind of stuck here.... I wish I had an oscilloscope (and knew how it works), but I don't...
Please give any advice, I am about to throw the thing off the window (after having broken it into little pieces with a heavy hammer, hahahah), never have seen such a notoriously malfunctioning machine in the 20+ years of my hobbyist repairing history...
Standby alert 
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