Apply for the 4 year course.
Usually these courses start as one cohort with a common initial pathway and formally split for years 3/4. This means that there is usually some flex to switch between the two degrees - but it is more difficult to switch up (BSc to MSci/MSc etc) in terms of SF funding as in their eyes you are switching programs mid-degree and extending your degree. If you start an M degree then you can switch down for a shorter degree far more easily.
Most combined BSc/MSci programs have the same entry requirements so it makes no difference in terms of offer, but usually the M degree will have fewer initial applicants so this may increase your chance of an offer. Just be aware that for M degrees you need to show in your PS that you are interested in more profound things than the basic degree, that you have a deeper interest/knowledge/proven aptitude for the subject and are interested in current research areas within the discipline.
And btw, an undergrad M degree is not the same as a postgrad MSc, so this isnt a sneaky way of getting funding for a postgrad degree - but it is a bridge to postgrad that would put you in a better position to apply for postgraduate funding for a PhD etc, or applying for a graduate research job in industry, than just having a BSc.