Call your power company right now and report an outage

This is a standard failure mode in North American split-phase power (as specified in El NEC, the continental code book.

The "Some circuits going above 120V" is a dead giveaway for Lost Neutral.

So this exact thing happens here (Asterisk). (while it uses DC, it's designed to show how AC power systems work, and it's accurate).

https://www.youtube.com/watch?v=LJvyb_WujZg

The vast majority of the time, lost neutrals are due to the service drop wires from the utility. It is typically delivered with triplex wire (2 insulated conductors wrapped around a bare wire which both physically carries the weight, and carries neutral). The wire whips in the wind for 30 years, the carrier/neutral takes the brunt of it, and aluminum has no fatigue limit. (nor does copper).

This service drop wire is the power company's responsibility, so they should fix it fast, for free. Assuming they aren't overwhelmed with repairing damage from an ice storm :) Ours came out in an hour on Sunday.

Now that asterisk. It's not quite like in the video, because your house has a "neutral-ground bond" which ties neutral to your local ground rods to the earth. So some neutral current is able to pass through the earth and back to the pole's ground bond (or your neighbors'). This means that voltages remain somewhat sane, typically no worse than 80V/160V.

Costa Rica uses El NEC. It's the American code, but in Spanish... and probably a few editions behind (heck every US state is at least 1 edition behind - NEC started piling on very costly requirements starting in 2014, and states are dragging their feet more than ever.)

Wire break? Call your power company right now and report an outage

This is a standard failure mode in North American split-phase power. The overhead wire whips in the wind for 30 years, the carrier/neutral takes the brunt of it, and aluminum has no fatigue limit. (nor does copper). This service drop wire is the power company's responsibility, so they should fix it fast, for free. Assuming they aren't overwhelmed with repairing damage from an ice storm :) Ours came out in an hour on Sunday.

The "Some circuits going above 120V" is a dead giveaway for Lost Neutral. Here's an example of the behavior there, rearranged a bit as a demonstration. (while it uses DC, it's designed to show how AC power systems work, and it's accurate). The asterisk is that a properly installed house will have local grounding rods, giving neutral current an alternate path through the dirt to other homes with ground rods or the supply transformer. Ground rods may be Ufer or water pipe connections.

Another failure is a Lost Hot. This is an odd one, because it almost completely breaks 240V appliances, and it should break 120V loads on the broken phase/pole. However in fact, it connects all your 240V loads together in series with all your 120V loads on that phase/pole. So voltage will mysteriously recover when a 240V load cycles on; the water heater being a major influence here.

Your reported 13-24V voltage drop cannot be attributed to the 600' run of #4 wire. The 1400W load you mention should cause only about 3.4% drop (4 volts), and the 1400 + 1200 watt loads together should only cause about 5.7% drop (6 volts). So no. It's not the voltage drop.

Your next step, if you're comfortable opening up your panel, is to take your voltage readings at the top of your main breaker (as close to the meter as you can get without breaking seals). That will most accurately reflect the delivered power.

If the power drop is local in your panel, look for loose connections. It was recently discovered screw torques are critical even on small circuits, and NEC 2014 now requires use of torque screwdrivers on every device which specifies a torque. An often overlooked place is the neutral bar on your panel. A suspect connection is far beyond torquing it now. It needs to be pulled apart, inspected, cleaned up if necessary, and cut back to copper which has not been annealed by heat.

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There is no such thing as "not subject to Code". There is such a thing as "no pre-accident inspections" but the two shouldn't be confused. Costa Rica uses El NEC, the continental code book and that is a matter of Federal law. Thus there is a positive reference for "correct wiring" that conforms to USA practice (except for being in Spanish :)
There are many "Give em plenty of rope" jurisdictions" where inspection and enforcement are thin. That seems good, but it's the opposite. it means selective and politically motivated enforcement. Somebody offends a local official or someone dies or just says the wrong thing on Youtube, and they're out there with the codebook and a micrometer. Thus, taking advantage of local political conditions to evade compliance is a dangerous game - and a private decision between you, insurer, mortgage lender, AHJ, prosecutor, the bereaved, and your deity.