The synth rack is the electronic equivalent to a physical rack of rack-mounted synths, and is Sonar's method of loading and working with soft synths.
Different soft-synths have different capabilities, and some soft-synths are what is called 'multi-timbral'. That means that they have the ability to handle reacting to multiple midi channels (1-16), as well as have multiple different sounds loaded and responding separately to midi data by having each sound react to a separate midi channel.
The advantage to the above is that it only 'costs' 1 loaded instance of the synth engine to be able to process these multiple independent sounds, and that can be a significant memory savings, depending on the synth.
It is up to the user to manage the amount of memory used in a given project by instruments and loaded sounds, for synths in the synth rack, and if available computer memory is on the low side this can take some thoughtful planning. CERTAINLY, deleting unused soft synths is always a good thing, for example. On the other end, choosing a particular string section sound might require a massive amount of memory, and one may have to choose wisely when loading up the sounds to be used for a given project.
Many soft synths include the memory requirements for their presets, so you can get an idea of how costly using a particular preset will be.
IF you are running in a 32-bit Windows environment, you will have a grand total of around 3.5 GB of memory, as there is about .5 GB used in OS overhead, out of your 4 GB.
IF your computer can support additional memory, you may consider maxing out the memory, HOWEVER you would then need to reload your entire computer to switch to a 64-bit Windows environment, and great care needs to be taken when doing this, so that you KNOW that whatever drivers you need for your audio interface and such are supported in 64-bit Windows, and you must also back up all of your data files and make a list of all programs that you would want to carry forward, since you would need to reinstall them all over again.
But IF you can go to 64-bit Windows, and get and use 8 GB of memory or more, this would make a GIGANTIC improvement in your system being able to handle more complex projects, and well worth doing.
A 4 GB memory strip runs around $20 these days, so if there is an open slot on your motherboard, and it supports 8 GB of memory, that is a VERY inexpensive and great bang for the buck improvement (if drivers are available and you don't mind the hassle of reloading everything in a 64-bit Windows).
If your current memory configuration is 2 2 GB memory strips, to get you to the current 4 GB, then you would want to pick up 2 4 GB memory strips, for around $40, or, if your motherboard supports single strips of 8 GB you could pick up a single 8 GB strip. (if your motherboard supported more than 8 GB you could then plug in the single 8 GB strip and leave in whatever other strips there are slots for, giving you a total of more than 8 GB, depending on the size of the additional strips).
Again, if we knew what your computer specs were, we could help you figure out what choices you had. Knowing them helps us to help you. :)
And, there are deals at places like Micro Center, where you can frequently pick up an AMD CPU and they literally will GIVE you a motherboard to go with it, as well as offer several others to choose from for nominal fees (like this board for an extra $10, or that one for an additional $20, etc.) You could pick up an AMD CPU and motherboard for between $100-$150 that would give you the capability to load up as much as 32 GB of memory and also have enough horsepower to run Sonar in a 64-bit environment, for example.
So, hope the above is of some use to you, and we stand ready to try to help you.
Bob Bone