4gb of ram are overkill for the tasks mentioned.
Macbook Airs in 2015 come with 4gigs standard and they run the stuff you mentioned with complete ease at rock bottom memory pressure. No unresponsiveness or other lag.
Whatever iMac you are refering to is not slow because of insufficient memory.
What an interesting debate we have going on here!
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Anyway, I'm more inclined to agree with Miester. Now my inner techie automatically wants to recommend 8GB because I think that
should be Apple's minimum and that is the
minimum I would accept, but I have to remind myself that most people aren't me.
DoFot9 said that Office, Skype, and Chrome each take 1GB of RAM and that simply isn't the case. I use all those products and Office (even if running multiple word documents and spreadsheets
and presentations) has rarely, if ever, exceeded one GB for me. Skype only use a few hundred MB for me. As for Chrome, it's possible depending what you're browsing, how many tabs, how many extensions etc.
I would want to hedge my investment with 8GB, but it's definitely not necessary. My mother got a Mac Mini (2012) and I had her just get the 4GB base model because she is a light to average user and didn't want to pay Apple's premium on RAM. She just does the type of things mentioned here and her system uses about 3.4 of 4GB of memory constantly. That works even for light iPhoto use. You're system won't lag unless it's severely low on memory (red memory pressure). If your 4GB system is weighted down by Office and Skype than there is something wrong. Chrome I can't comment on.
Now, I will say this. I like like to have room to grow. Since Apple has decided to lock the RAM in for life I would be inclined to recommend 8GB if there is any chance you might expand your usage. For example, running a VM, heavy use of iPhoto, or heavy use of other photo editing software (like pixelmator) can be burdensome on 4GB, but is still reasonably possible. I used iPhoto heavily on my 4GB system and ran VM's with 1GB of RAM allocation. The VM did cause lag, but it was still usable.
I apologize, that was far more wordy than I intended.
Short Version: 4GB is sufficient for everyday tasks and will not cause lag. I'm inclined to recommend 8GB to allow for usage growth and because I'm a techy. If I had to choose between more RAM and an SSD I'd probably the RAM
for me, but I think the average user would benefit more from the SSD for day to day tasks.