I bought a tumble dryer a year or two ago and I found that there was huge variance in the prices of different models with not really much correlation to the features etc.
Having a Which best buy stamp often bumps the price up in itself, because a lot of people will just automatically go oh yep OK that one, good. But there may be other extremely similar models which simply weren't tested by which so they didn't get that rating. Also many white goods will fluctuate in price quite massively -look on price tracker type websites that can show price history. Amazon has Prime day soon, it's worth looking on there for appliances - we were happy with their service.
Usually the "series" is like a generation so the Series 6 may be 2 generations newer than the series 4, which generally means improvements across the board. Since they are the same brand, it would be easier to compare - you could go to any electrical appliance showroom (preferably an independent one, not a chain, but a chain will do) as the staff there will be trained by Bosch reps and will know the difference. Go in and ask them what's the difference between series 4 and series 6. Of course because they are sales they will push the more expensive model, but a good salesperson can tell you what you're getting for the extra £££ (whether they dress it up as desirable, essential etc) which can help you decide.
I found this channel hugely helpful, this is the kind of independent place I mean:
https://www.youtube.com/@lords-electrical
I asked a question on one of the videos and he made another video answering my question a day or two later - brilliant!
A good way to find local independent retailers for any type of item is go to a website (e.g. bosch website) and look for stockists by area, then find anything which isn't a recognised chain name and go and check them out.