It is the TIFF format itself that is limited to 4GB. TIFF files larger than 4GB are invalid. I have read about some new libtiff libraries (4.0b) that are in beta testing that support TIFF files larger than 4GB, but the officially released libtiff libraries are limited to 4GB TIFF files.

I actually think this might be a bug. When writing to Imagine format, a '.img' file should be created with an associated '.ige' spill file. When the file size is larger than 4GB, ENVI does not appear to write the .ige file but when under 4GB it does. I believe you should be able to write Imagine files larger than 4gb. I have created a bug report for our developers to consider this.