Q: Is it ok to mix 2 GB and 4 GB in an iMacbook Pro (Early 2011)?

Yes. Just note that macs are picky about memory and y should only get memory from place that guarantee it will work with your Mac. Crucial and OWC/MacSales are such places. Memory that match the Apple specs do not always work

Probably, if the memory RAM is of best quality; but the computer (if identified correctly)

can use up to 16GB Total, or 8GB in each of the two memory slots. Matched pairs of

equal capacity can help the computer be more efficient by design...

{info from free specification utility app http://mactracker.ca for your Early 2011 MB/Pro}

Maximum Memory    16 GB (Actual) 8 GB (Apple)

Memory Slots    2 - 204-pin PC-10600 (1333 MHz) DDR3 SO-DIMM

• MacBook Pro: How to remove or install memory - Apple Support

There are three MacBook Pro models identified by the Early 2011 generalization.

A 13-inch, a 15-inch, and a 17-inch; each has more specific detailed identifiers;

as such to included those would be more helpful for sake of accuracy.

Because graphic processor in MBk/Pro8.1 13-inch model shares standard RAM

from the main memory capacity, you can help graphic performance by upgrading

standard RAM. The larger display models (MBkPro8.2 15-in +  MBkPro8.3 17-in)

use different upscale graphics, and those do not use shared main memory RAM.

• MacBook Pro 13"-15"-17" Early 2011 - View Memory Options - macsales

Each of the three MacBook Pro Early 2011 models have certain specs; however

they do share memory part number and upgrade capacity totals Aftermarket part

from OWC macsales and from crucial companies (see .com site) specific to these

models are guaranteed to work in these MacBookPro models and tested in them.

{requested relocate to MacBook Pro portable section, instead of Mac Pro desktop}

Good luck in this matter!