I suggest for now looking at the clock is a good idea. When a LO is on a good routine which is working watching out for cues is great as you can judge if LO has had an extra-stimulating morning and needs a slightly earlier nap or is perhaps coming down with a cold or something. But when the routine goes off track I find it is more helpful to watch clock.
At this age cues can be difficult to read, a yawn may mean boredom and needing a change of scene or activity rather than tiredness, being handed a different toy to hold or being moved into a different room can easily extend the A time and stop the misleading yawns. You are also right though, the 4 month sleep regression is a time when many LOs start taking short naps, my DS did it too, for us it started at 3.5 months and continued until 5.5 months.
To be honest it looks like you might have reduced the A time too much. You say he was previously doing 1hr 45 and I see that your pre-CNing EASY has a first A time of 2hrs so he was likely ready to increase to 2hrs and 2hrs 15 (perhaps 2hr 15 just on the first A time and perhaps not all day) however your more recent EASY shows a first A time of 1hr 45 (so it is less than before the problems started) and the second A time of 1hr 35 which is likely not a big enough increase to help him be properly tired for that nap.
The early night waking looks more like UT to me rather than OT. Often these early evening NWs are down to OT but I would expect to see it happening after too long a last A time and LO crying abut a bit easier to settle down - I can't say for certain but it looks like he's taking it as another CN and wanting a long A time after it.
See how you go on 2hrs (and longer on that first A), Tracy said in her book to keep them awake the full A time and to do a fan dance if necessary. You need him good and tired to help him transition between sleep cycles or to make it easier for you to help him resettle. The temptation is to reduce the A time following a short nap but in my experience this just perpetuates the UT issue. You can either count the next 2hr A time from the end of the short nap or you can do as Tracy described in her BW book and set out the day, knowing when the nap times are and make LO wait for the next nap time - this does mean a very very long A time but it can often mean getting back onto a routine faster too. It's up to you.