As a further clarification:-
The dry boiling point is the boiling point of the new (pure) brake fluid.
The wet boiling point is the boiling point of the fluid when it contains 3.7% (by weight) of water. This is typical of a fluid when it has been in use for 2 or 3 years.
The viscosity limit is the MAXIMUM viscosity allowed at -40C.
DOT 5.1 has higher boiling points than DOT 3 or 4 fluid, but it has a lower low temp viscosity to ensure that systems like ABS, ESP etc work correctly when it's very cold.
Given that DOT 5.1 costs about £1 per litre more than DOT 4, I think it is worth using DOT 5.1 all the time. It can be difficult to find but Halfords stock it.
Interestingly Castrol do not sell a DOT 5.1 fluid in the UK. They sell a "Performance DOT 4" fluid, which has higher boiling points that the DOT 5.1 spec requires, but does not meet the low temp viscosity requirements. Boiling points are 260C and 180C, low temp viscosity 1200. They also sell a "Low Temp DOT 4", boiling points 265C, 175C and low temp viscosity 650, meeting the ISO 4925 Class 6 which prioritises low temp performance at the expense of a slight reduction in boiling point.
For further reading, in case of severe insomnia, see the following:-
https://law.resource.org/pub/us/cfr/ibr/005/sae.j1703.1995.htmlhttp://www.shell.com/content/dam/shell/static/chemicals/downloads/products-services/shell-brake-clutchfluidsbrochure.pdf