Based on the information I am given so far, I can only presume that
maximum likelihood estimation was intended as the solution method. (After all, we are not allowed to assume anything about the prior distribution ... what else can we possibly do?)
So, with regard to MLE, let n be the total number of tiles and k be the number of blue tiles. The number of ways of choosing 4 blue and 8 yellow tiles is f(k)=C(k,4)C(n-k,8), and we need to find the k that maximizes this. (Here, C(a,b) denotes
a choose b.)
Now the ratio of successive terms is f(k+1)/f(k)=[C(k+1,4)C(n-k-1,8)]/[C(k,4)C(n-k,8)] = [(k+1)(n-9-k)]/[(k-4)(n-k)], and so find when f(k+1)/f(k)<1:
[(k+1)(n-9-k)]/[(k-4)(n-k)] < 1
-k^2+(n-10)k+(n-9) < -k^2+(n+4)k-4n
5n-9 < 14k
(5n-9)/14 < k
So f(k+1)/f(k)>1 when k<(5n-9)/14 and f(k+1)/f(k)<1 when k>(5n-9)/14. This implies that the sequence f(0), f(1), ... , f(n-1), f(n) is maximized at the first k value when f(k+1)/f(k)<1 (that is, ceiling((5n-9)/14) ).
For n=100, this value is ceiling(491/14) = 36, so the drawing of 4 blue and 8 yellow is most likely when there are 36 blue and 64 yellow tiles in the bag to begin with. After drawing, this leaves 32 blue and 56 yellow, so the probability the next draw is blue is 32/88 = 4/11.
For n=30, this value is ceiling(141/14) = 11, so 11 blue and 19 yellow. After drawing, 7 blue and 11 yellow are left, so the probability of drawing blue is 7/18.
As the number of tiles goes to infinity, the proportion of blue tiles in the maximum likelihood estimate approaches 5/14, so the limiting probability of drawing blue is 5/14.