Sorry if this gets a bit long winded, but what you want can be done, you just need to understand the process(es).

Assuming that you are dealing with non carbonated cider, if you start with a SG of 1.050 you can pasteurise to stop fermentation at 1.020 to get about 4% ABV. The formula is simply ABV=(OG-FG) x 131.25. So, (1.050-1.020) x 131.25=3.9% ABV. Alternatively, pasteurising at 1.010 will give you about 5% ABV and a medium sweet cider.

Or, as suggested in the post above, let the cider fully ferment, dilute with apple juice to taste or until you get 4% ABV (i.e a ratio of 10:7.5), then pasteurise the whole lot. Beware that if you don't pasteurise, the sugars in the plain juice will start fermenting because there will be residual yeast in the original cider and you will end up with something like 10 volumes of CO2 carbonation instead of the normal 2.5 volumes.... volcanoes!!!!

I am not sure if mixing your cider with chemically pasteurised juice would overcome this.

However, if successful the result from these approaches will be somewhat sweet because you should end up with sugars that are not converted to alcohol by the yeast. But, your friends may like it sweet as well as lower alcohol.

You can heat pasteurise or chemical pasteurise. To heat pasteurise, simply bottle and heat the bottles up to 65C-70C in a waterbath for 10-20 minutes to kill the yeast and let the bottles cool down (Pappers post at the top of the forum describes how to do this or for more information look at my post of 25 September on the same subject).

If you want low alcohol, carbonated cider, then bottle and let fermentation continue until some carbonation takes place before you pasteurise. It can be a bit tricky knowing when to do this as you need to allow a SG change of something like 0.005 to get 2.5 volumes of CO2 carbonation, so you need to have some idea of the rate of fermentation in order to estimate when the 0.005 change has taken place. Alternatively have some cider in a plastic soda bottle and when it is firm, you probably have enough carbonation (this is a bit imprecise but works O.K.).

I understand that an alternative to all of this is to rack at 1.020 then again at 1.015 and again and again with filtering. I have never done this because it seems too hard, but the idea is to denude the yeast population so that no further fermentation takes place. You can also suplement this process with chemical pasteurisation. Perhaps someone familiar with sorbate and sulphate for pasteurisation can contribute.