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The softness is almost certainly a limitation of the lens, i.e. it's not sharp at 800 mm. High quality super telephoto lenses are very, very expensive... e.g. Canon's 800 mm f/5.6L lens is *double* the price of a Tak FSQ-106!
At 1/4000 sec, it's very unlikely to be blur from sidereal rotation. It doesn't look like rotation blur because the spots are round (not elongated in one direction), and the edges don't show any trailing blur.
By the way, approximate calculations for trailing just need a bit of arithmetic. The sky appears to rotate at around 15 degrees per hour (15 arc seconds per second), so in your 1/4000 second exposure it will rotate 15/4000 = 0.00375 arc seconds. The sun is around 30 arc minutes in diameter, and in your image it is approximately 800 pixels high, which gives a scale of 30*60 / 800 = 2.25 arc seconds per pixel. Thus, during one exposure the sun will have moved 0.17% of one pixel - i.e. there is definitely no trailing occurring here.
One last thing... 1/4000 sec ISO 100 f/16 is still *VERY* bright - so please be careful and protect your eyes. Using a Thousand Oaks 99.999% blocking solar filter, I get exposures of 1/250 sec ISO 100 f/4 - i.e. your solar filter lets in 256 *times* more light than a typical safe-for-visual-use filter.
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