You should store the command java -Xmx4096m -jar path\to\SweetHome3D-4.1.jar in a .bat file. For example, name it SweetHome3D-4GB.bat.
But if you want it to work, you have to change two things in this command:
- the path to the java executable: as you have two versions of Java installed on your computer, you have to tell the system which one you want to use. If you installed Java 64 bit in the standard place, that must be C:\Program Files\Java\jre7\bin.
Therefore, you should replace java by "C:\Program Files\Java\jre7\bin\java.exe" (notice the mandatory quotes because there's a space between Program and Files words).
- the path\to\ part in front of SweetHome3D-4.1.jar file: if you kept it on your desktop, it must be something like C:\Users\Administrator\Desktop\ or C:\Users\userName\Desktop\ depending on your user's name or whether you're the administrator of the computer. If you stored it in Sweet Home 3D standard install directory, that must be C:\Program Files (x86)\Sweet Home 3D. In that second case, path\to\SweetHome3D-4.1.jar will become "C:\Program Files (x86)\Sweet Home 3D\SweetHome3D-4.1.jar" (notice the mandatory quotes again).

All in all, if you copied SweetHome3D-4.1.jar file in Sweet Home 3D installation directory, your SweetHome3D-4GB.bat will probably contain: Hope you can handle it [:)]