OK, I read it deep in the night and was a bit gruff. Sorry for that :)
Well the best would be to not use Windows XP anymore as Microsoft officially stopped supporting it:
http://windows.microsoft.com/en-us/windows/end-support-help

This means Windows XP doesn't get updates anymore, security holes will not be fixed anymore and system components do out date more and more.
It is a high risk to use Windows XP since Microsoft stopped support.

And if you want to use 4GB or more you need to use a 64bit version.
WftO supports Windows 7, 8.1 and 10.
If upgrading from 32 to 64bit without "wiping out computer" is doable depends on multiple factors, I remember it was mostly not easy with Windows XP and a 32bit license does not grant a 64bit license.
I hope this page answers questions about this topic:
http://windows.microsoft.com/en-us/windows/32-bit-and-64-bit-windows#1TC=windows-7

I never did such an upgrade my self and I can only provide limitted help on this matter.
We are here to fix issues with the game but can not always help with issues users have with their system.

I still hope you find a way to play or maybe one day you can afford a new rig without all these limits.

Either way I wish you best luck :)

Short answer: It is a game from 2015 and not from 1995.
It is using a game engine Unity3d which it self will require a lot of this RAM for all the nicer graphics you can see.
Also mechanically the game is more complex in some ways and the AI is more advanced.
Also nowadays memory is always enough on most systems, so that modern frameworks like to rather allocate more memory if this is a benefit to runtime.

Your rig + OS is from more than a decade ago, not even Microsoft supports WinXP anymore and you even have the 32bit version.

I am not sure what else to say, I was just trying to help so you can still run the game even below minimum requirements.

Well the top three things take 1GB of memory away from your 3.5 GB usable RAM so that 2.5 RAM remain.
(1 - Opera can be closed, 2 - AMD Evolved can be closed, 3 - Windows Antimalware you might want to keep but can be closed)
WftO requires 4GB RAM to work on all included maps correctly, when you play custom maps maybe even way more.
With that you are still below minimum requirements.
So what ever you try to save is still too big for your RAM unfortunately.
What is it by the way?
I guess it's not a small map.

Hello Tom Bushell,
we are believing meanwhile this is a general issue with the new OSX version "El Captain".
Like the last link stated here
https://brightrockgames.userecho.com/topic/1084024-game-not-working-in-fullscreen-mode-os-x/
It seems there is some issue with their display modes.
Such issues were not seen in versions before "El Captain".

I hope that Apple is patching this in the near future.

Hello Gigi Miner,
as far as I can see this was a temporary issue with a hotfix we released.
It was only happening for a few hours until we spotted, fixed and patched it.

So it should work now correctly again.
Please let us if not.

Cheers

So I even played the map my self to the point where I claimed the "central corridor".
Despite this map being almost unplayable because of too many objects placed causing lag,
I was able to claim the center without any issues.

Or isn't this the "central corridor" you were talking about?

Hello Julianb3000,
any chance that the tiles in the center have hidden Blade Loti on them?
This makes workers avoiding these tiles, until you rally units on that tile which makes the Blate Lotus pop out.
I might take a look at this map as well.

Cheers

If you switch to anything equal or higher to windows 7 64bit you should be fine.
A 64bit OS is important, if you want to be able use over 3.5 GB.

Cheers

Uhhh this looks really messy.
I have never seen something like this so far and I am pretty sure this will not happen on every map as we tested this feature on multiple.

Which map is it? Can it be found on Workshop?
Did you encounter this on any other map as well?

Can you restart the game, import it again to trigger the problem and then once it occured, quit the ggame and upload your output_log.txt,
like specified here https://forum.subterraneangames.com/threads/please-read-how-to-report-a-bug.4689/
It's important to always grab the log before restarting the game after a problem occured, because it will be recreated on each launch.

There must be anything going utterly wrong which caused the problem.
The log might allow us to figure out what it was.

Cheers

There are quite some other useful improvements as well.
Have fun! :)