At the risk of being labelled (entirely fairly) a dull history spotter-type...
Seniority is the trump card in all cases, bar for 617 (and, if we get an MPA, 120 which is in the same boat - no pun intended), which is treated as a special case because it was awarded its standard ahead of the 25-year accumulated service point in recognition of wartime service.
I am guessing that 1(F) will be making a reappearance fairly soon - it is the second most senior squadron in the RAF after II(AC), and IV is not far behind. My assumption - which will probably be entirely incorrect - is that 1(F) will get Typhoon, as it is the most senior available numberplate, and the rules decree that the most senior available numberplate will reform at the earliest opportunity, give or take some degree of latitude in terms of association with a particular role if there is a choice of numberplates (i.e. you might reform 25 on the A400M, since it is senior to 70, but 25 is not 'role associated' with AT).
IV has presumably taken over the numberplate at Valley for one of two reasons - either (a) because it is being kept going until JSF (or similar, see PPrune, passim) whereupon it will become the first light blue JCA unit or (b) because there is a more machiavellian plan at hand, which is to tie IV's numberplate up with the Hawk, thus clearing the way for the reformation of a slightly less senior unit - for instance, don't rule out the possibility of 19 reappearing as the final Typhoon squadron. In truth, only one squadron has ever come close to benefitting from 'type b' shenanigans, namely 74 - which the Air Staff attempted to reform on numerous occasions during the 1970s, only to find that they couldn't generate a sufficient reason to get around the barrier of 74's relatively lack of seniority. Pre-fiscal disaster, it did look as though the numberplates in the way of 74 becoming a Typhoon squadron were being put into use so that we could have an FJ Tiger Squadron again (the RN may have to reform 804 to get that happening).
There is a slight complication in that in theory, the rules do not apply when granting a numberplate to a Reserve squadron - which is why 76, which is well down the list of seniority could reform, drawing upon local links between that squadron and the area around Linton-on-Ouse.
As a final point, seniority has been calculated in some odd ways in the past (including, prior to the 1960s, via the awarding of points for having a VC awarded to a squadron member, points making prizes) and 'most senior' does not mean 'oldest'.
If you've stayed awake this long, well done.