It's no secret that in the early days of computers, they had only 1-4MB of RAM. I found an article stating you needed only 4MB of RAM to run Word, Excel and PowerPoint simultaneously. How was that possible, considering that nowadays you need at least 2GB, 1000x times more RAM to do the same thing? Sure, the new apps have more features, but at the core it's the same thing.
How did heavy software in the early days managed to run on 4MB RAM computer?
It wasn't heavy.
That's it. There is no magic to it. Your question is based on a false premise.
I remember that my mom was using my dad's old IBM PC/AT with 512 KiByte of RAM and a 20 MiByte HDD well into the 1990s for word processing using Volkswriter and then later StarWriter. Considering that she was able to run a word processing program in 512 KiByte of RAM, I have no trouble believing the claim that you can run 3 programs in 8 times that.
The first word processors were written for computers whose RAM was measured in bytes, so why would it be surprising that you can run one in a million times that?
Microsoft Word for Windows was by today's standard a very small program. Its size is 13.07 MB on installer disks (1.44 MB each), which you can download to check for yourself.
Microsoft Word 6.0 for Windows ran first on Windows 3.1 (released April 6, 1992) and looked nothing like Microsoft Word do today:
Internet wasn't available, so updates to software where shipped in floppy disks. But it was a very good implementation of Word processing, and we still use the same keyboard shortcuts today as we did back then.
On MS Excel, when making advanced calculations in cells, my computer took a break. I could go to the next building and grab a coffee, and when I came back, the PC still wasn't done computing.
Computing power was scarce those days, and it wasn't always pleasing being an information worker back in 1992.
As for memory consumption today(!), you'd need 150 MB to run MS Word, MS Excel and MS PowerPoint in the same time. That's a lot more than your 4 MB back in the days, but I have 16 GB available, so that's fine. What worries me though is that Google Chrome with this tab only open consumes 600 MB of RAM, telling me that it has nine ongoing processes. Time to look into that, but that's another story.