I believe you are limited to 2GB memory per application in a 32-bit OS, even if you have 4GB RAM available to the system.
 
So 64-bit can help you avoid crashes that may be due to hitting an out of memory situation, particularly with hungry plugins and large sample libraries.  It will also let you access the full 4 GB, because on a 32-bit system you will only see about 3.5 GB available.

x64 bit versions of Window 7,8, and 10 require a MINIMUM of 2 GB RAM to run, x32 require a minimum of 1 GB.
No OS is going to allow applications use all physical memory because they OS needs to reserve and use some for itself. Hence, you can only maybe access 3.5 GB out of 4 GB, and that's actually pretty unlikely to happen with all of the background services, sharing memory with graphics cards and chips, and things like virus protection going on.
 Not to mention sharing 2 GB of RAM with any one App. with an x32 bit system is an extremely generous amount of sharing.
 
A x64 bit system with only 4 GB RAM might be OK for office work, but very sub par for reasonable performance for a workstation such as a DAW, and utterly abysmal for using hungry plugins (needs a strong and stout 4+ core CPU and fast buss speeds) and large sample libraries (needs ASIO drivers, lots of duel channel RAM and fast buss speeds).
 If not than things are NOT going to be going smoothly with a starving SONAR if per chance it goes at all.

Thanks guys.  I'll need to get extra RAM anyway as 4GB is a bit low.  BUT..... having said that.  The laptop I use for Sonar has no other apps installed.  I have one laptop (this one) for internet, Itunes, Office etc.
My music laptop only has Sonar installed to try help regarding the whole RAM, CPU resources etc. 
I wish i could afford a solid state drive but they are still very expensive.