I am currently doing four a-levels and have received offers from all of my uni's for Mpharm 2011 (Manchester, Nottingham, Kings, LSOP, Bath), all offers specify that i get certain grades in three a-levels indicating that they do not really care about my fourth a-level. so as long you have 3 full a-levels of relating subjects you will be in no way disadvantage from someone who does four; such as myself.
with such circumstances as you have mention i believe these to be a credible reason that universities will recognise if you were to not perform as well as you believe you could. if you are to contact your universities directly with this information i am sure this will be considerate and submit offers accordingly. My opinion is that your predicted grades are good enough for universities places, if you feel that you may under perform i would recommend taking only 3.5 as i believed 4 would give me an advantage which it has not really done.
All the best to you and your family