Are you taking RAW images ? If so you can process the 4 images in Adobe Camera Raw converter (ACR) [to 8 bit RGB] .TIF file format.
In ACR you can concurrently adjust the white balance so that ALL are same color temperature, Red Cyan to -52, Blue Yellow to -25, adjust the vignetting to +100 and mid point to 0. Those setting should fix the apparant stitch lines.
Panoweaver 4 does stitch 8 bit TIF's. But not 16bit images. 16bit gives better color depth.
The small black square in the top of your spherical image is actually a black area across the top of your stitched image. You can clone that black area in with adjacent sky colors and the black square will go away.
Typically we are resizing our stitched images down around 400 - 600 kb for dial up folks. Most all of our full screen java or qtvr's are 1.5mb for broadband visitors.
Canon 20d/ Sigma 8mm/ Kaidan Kiwi990.
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