There's no such thing as DOT4.1 unless I've missed something?
DOT 5.1 is the one to go for, it does not absorb water any more than DOT4, it is just a higher spec fluid (higher boiling point). Castrol Response and Super DOT 4 is not better than any old DOT 5.1 (Eg Comma etc), I've tried them all on track and any variant of DOT4 gave me a spongey feel afterwards; with DOT5.1 I have never had any problems so it's back to a year/2yearly brake fluid change.
Dry boiling point Wet boiling point
DOT 3 205°C (401°F) 140°C (284°F)
DOT 4 230°C (446°F) 155°C (311°F)
DOT 5.1 270°C (518°F) 191°C (375°F)
DOT5.1 is the next step up in spec over DOT 4. The ".1" part of the fluid was added due to DOT 5 already being released but that is not often available and car manufacturers never took to it as it's a completely different type of fluid that is not without problems for brake systems (silicone-based, unlike all other DOT 3/4/5.1 fluids).
The next step up from 5.1 is stuff like MOTUL and the ultimate is Castrol SRF @ £40/ltr.