MacBook Air 13.3″ - 4GB RAM Limitations

What are the limits when we are talking about applications running at the same time?

I have not encountered any, since I can load every single application that exists on my MBA and I have encountered no limitation. That includes iMovie, iPhoto, iTunes, Keynote, Safari, Pages, Numbers, BBEdit, OpenOffice, Photo Booth, Messages, Maps, FaceTime, iBooks, PCalc, certain other proprietary software, and many, many others, simultaneously.

Can you give me some examples based on your own experience (e.g. how many tabs opened in browser?

As many tabs as you wish. Any limitation you encounter is more likely to be that of your Internet speed. For example, Safari 7 suspends execution of performance-demanding Internet content under certain circumstances, to maximize a Mac's battery life and other system resources. You can watch a demonstration of it in Apple's presentation some months ago.

which applications were running exactly?)

I use Safari, exclusively.

where the MBA's 4GB RAM handled the situation smoothly without any hiccups?

Yes. Flash memory goes a long way toward masking a paucity of RAM. Once you buy a Mac without a traditional spinning HD, you'll never want anything else.

Bear in mind millions of people are using iPhones and iPads that have no idea how much memory they have, nor do they care (in fact they have very little RAM). That's the future of the Mac product line.

Memory management improvements in Mavericks helps as well. When Mavericks was introduced, Apple claimed an effective RAM utilization increase of about 50%, a claim that I have empirically proven to be accurate, perhaps slightly conservative. What that means to you is that 4 GB RAM will act as though 6 GB were actually installed, if it weren't for Mavericks' memory compression algorithm.

typing from a 2GB RAM tablet with 4-core ARM processor - having used a number of tablets with less memory and having to flush RAM or close programs is a factor. This one has SSD and paging, also a 250GB USB3 SSD plus 64GB flash card

The dual core with small 128GB SSD in low end MBA can be limiting. I would try to get the 256GB SSD model at least.

Also the differences between a lean mean clean OS and one that has gobs of stuff stuffed in can have a big effect and reason people complain about running slower...

There is one consideration to take into account, to further complicate your decision: while the MBA's flash memory is replaceable (with difficulty, as it is not considered a user - replaceable component) its RAM is most definitively not - not without replacing the entire logic board at considerable expense.

While 4 GB should be sufficient, if you were to decide between electing an 8 GB RAM configuration vs. the 256 GB storage, choose RAM.

Apple uses custom form factors for the MBA's internal storage. I'm not aware of any aftermarket 256 GB options for the current production MBA at the moment, but it didn't take long for aftermarket upgrades to appear for earlier model MBAs. If and when larger memory capacities appear on the market, they are likely to be cheaper than electing the 256 or 512 GB options available at present.

On the other hand whatever RAM you decide upon now will have to suffice for the MBA's life. The 8 GB option is only an additional $100, money that you are likely to consider well spent years from now.

I've been using a 2011 MBA with 4GB for all my work for the past 3 years and I'm definitely in the power user category. Normally I'd have already replaced this computer with a newer model but I'm totally satisfied with its performance. Having said that, Apple will be releasing a new OS in the late summer and whether I'll still be happy is hard to say. And a year after that? If I were buying a new MBA today I'd choose the 8GB model for longevity but for now and probably for the next year or two this 4GB model seems safe.