We've debated brake fluid endlessly, with much passion on both sides. Personally, I'm a big fan of DOT 5
silicone fluid; but I wouldn't say that Castrol brand "LMA" DOT 4 is "wrong".
If you do run DOT 4 though, it needs to be periodically changed, as it will absorb moisture from the air which makes it boil easier and become corrosive to brake components. DOT 5 should be good for 10 years or more (as long as it doesn't get contaminated by rain or other FOD falling into the reservoir).
There is an old article at
https://www.buckeyetriumphs.org/s/Selecting-Brake-Fluid.pdf that I think fairly goes over pros and cons of both types. (In spite of the date in the corner, the article was written in 2002. Yes, you'll see my name mentioned.)
Don't be fooled by the "synthetic" label. All brake fluid is synthetic, it's not made from crude oil. The label is just a marketing ploy. Also beware of "DOT 5.1", which is NOT silicone. Although the specs are better, it still suffers from the same ills as DOT 4, namely sucking water from the air and eating paint when it gets loose. By DOT rules, if it's DOT 5 silicone, it has to say "silicone" on the front.
PS, many years ago, there was a problem with using non-Castrol fluids with British brake systems. The problem was real (although also somewhat contested, we didn't have the Internet to get into a proper flame war over it
). No one has mentioned it for a long time, so my theory is that the currently available seals ("currently" meaning the past 30 years or so) will work and play well with other DOT 3/4 fluids. Still, Castrol is a good brand and is readily available, so why take a chance?