Replacing heating elements on a hot water furnace

Frescard

I have an old Chromalox hot water furnace (model EDB-25-1), which has a broken heating element (no conductivity). I'm planning to replace it, but, having never done this on a furnace, am wondering if there's something to be aware of (apart from the obvious "turn off the power/water, drain the water", etc).
I've replace water heater elements before, and am comfortable doing electrical work, but just want to make sure I don't overlook something...
These are the elements, so it looks like a straightforward job: https://imgur.com/dN5zeL5

While I'm 99% sure that this element is broken, and the other ones are fine (all their resistances are within 10-11 Ω), the temperature of the furnace seems too low, to be caused by just one (of five) broken elements. The boiler temp (when running) is around 120ºF, and that doesn't seem right. Is it??? All elements are getting power on each pole, so the relays seem to be ok.

Ironman

Nothing special about replacing it: turn off power, isolate and drain the boiler.

As far checking the other elements: you need to assure that you're getting 240 volts to them between poles. The with the boiler refilled and pump running, measure the amp draw on each one. If they're the same size, the amp draw should be about the same.

Frescard

Thanks! That's good to hear.
(What do you mean by "isolate" though?)

It turns out, one of the sequencers was bad, too, so I'll need to replace that as well. Plus, one of the sequencers wasn't even connected to power (the jumper wire was just lying on the ground), so that means only 2 of the 5 elements were actually active! (That would probably explain the low boiler temp.)

While studying the wiring (and diagram) I was a bit confused though, over one sequencer that doesn't seem to do anything. It's #4, and I don't understand why it's there, or what it's switching: https://imgur.com/Jrm8QDU
Could I use this (at least temporarily) until I've replaced the broken #5?

Ironman

Isolate: valve off.

The image of the wiring diagram is too fuzzy to make out.

Frescard

The image is pretty hi-res. You can click on it to enlarge it.
Or use this one, or the attached one.