Suppose $m$ players are playing the game ($m=1000$) and there are $n$ distinct cards $(n=54)$. Let’s consider the game from the point of view of one player. Define the following events:

\begin{align}

A &: \left\{\text{No winning pattern in the first four cards}\right\} \\

B &: \left\{\text{No winning pattern in the first five cards}\right\}

\end{align}If we count all the possible winning patterns, we see that there are 18. Meanwhile, there are $\binom{n}{4}$ ways of picking four cards. The probability of no winning pattern is 1 minus the probability of a winning pattern:

\[

\mathbf{P}(A) = 1-\frac{18}{\binom{n}{4}}

= \frac{35137}{35139} \approx 0.999943

\]The approach is similar when there are five cards. Each of the 18 winning patterns uses 4 cards. The fifth card can be any of the remaining $n-4$ cards, so we have:

\[

\mathbf{P}(B) = 1-\frac{18(n-4)}{\binom{n}{5}}

= \frac{35129}{35139} \approx 0.999715

\]

Now let’s ask the question: what is the probability that there is a winning pattern using all five cards *given that there is no pattern in the first four cards*. We can calculate this using Bayes’ rule and the law of total probability:

\begin{align}

\mathbf{P}(\bar B \mid A)

&= 1-\mathbf{P}(B\mid A) \\

&= 1-\frac{\mathbf{P}(A\cap B)}{\mathbf{P}(A)}\\

&= 1-\frac{\mathbf{P}(B)}{\mathbf{P}(A)}\\

&= \frac{8}{35137} \approx 0.00022768

\end{align}where in the third step, we used the fact that $B \subseteq A$ and therefore $A\cap B = B$ (if there is no winning pattern in the five cards, then there cannot be a winning pattern in the first four).

The problem asks us to find the probability that if $m$ players play the game and nobody wins on the fourth card, exactly one person wins on the fifth card. Let’s call this probability $q$. Since each player is playing independently, there are $m$ different people that can win, with probability $\mathbf{P}(\bar B\mid A)$, and the other $m-1$ players must lose, each with probability $\mathbf{P}(B \mid A)$. So the probability that exactly one person wins on the fifth card is:

\[

q\,=\, m \cdot \mathbf{P}(\bar B\mid A) \cdot \mathbf{P}(B\mid A)^{m-1}

= m \cdot \left( 1-\frac{\mathbf{P}(B)}{\mathbf{P}(A)} \right)\cdot \left( \frac{\mathbf{P}(B)}{\mathbf{P}(A)} \right)^{m-1}

\]Substituting the numerical values found earlier for $\mathbf{P}(A)$ and $\mathbf{P}(B)$ together with $m=1000$, we obtain:

\begin{align}

q &= 1000\left(\frac{8}{35137}\right) \left( \frac{35129}{35137} \right)^{999} \\

&= \frac{8000\cdot 35129^{999}}{35137^{1000}} \approx 0.181356

\end{align}So the probability that exactly one person out of 1000 wins on the fifth card given that nobody won on the first four cards is about 18.136%.

### The general case

If we leave the expression for $q$ in terms of $n$ (number of distinct cards) and $m$ (number of players), we obtain:

\[

q = 1728m\cdot \frac{\left(n^4-6 n^3+11 n^2-6 n-2160\right)^{m-1}}{\left(n^4-6 n^3+11 n^2-6 n-432\right)^{m}}

\]If we fix $m=1000$ and vary $n$, we obtain the following graph:

When $n$ is small, it is very likely that many people will win. So the probability that *only one person wins* on the 5th card is low. But when $n$ is very large, it is very unlikely that anybody will ever win, so the probability that one person wins on the 5th card is low again. The peak occurs at $n=38$ (with probability 36.8%).

If instead we fix $n=54$ and vary $m$, we obtain the following graph:

The shape of the curve is similar. When $m$ is small, there are not many people, so it is not likely that anybody will win on the fifth card. But when $m$ is large, it is likely that many people will win, but unlikely that *only one person* will win. The peak occurs at $m=4392$ (with probability 36.8% again).

If we vary $m$ and $n$ together, we see that for each $n$, there is an ideal number of players $m$ that would maximize the probability (red dashed line in the plot below).

The red dashed curve has the equation $m = \frac{n^4-6 n^3+11 n^2-6 n-432}{1728}$.

### Numerical validation

To test whether the above formulas are correct, I ran a Monte Carlo simulation to estimate $\mathbf{P}(\bar B\mid A)$, which is the probability that a single player wins on the fifth card given that they did not win on the fourth card. I simulated a large number of games and only kept those that didn’t win after four cards. Then I computed the empirical fraction of the remaining cases that won on the fifth card. Here is my Python code.

# winning sets of indices
win_idx = [
[0,1,2,3],[4,5,6,7],[8,9,10,11],[12,13,14,15], # four in a row
[0,4,8,12],[1,5,9,13],[2,6,10,14],[3,7,11,15], # four in a column
[0,1,4,5],[1,2,5,6],[2,3,6,7], # 2x2 in first row
[4,5,8,9],[5,6,9,10],[6,7,10,11], # 2x2 in second row
[8,9,12,13],[9,10,13,14],[10,11,14,15], # 2x2 in third row
[0,3,12,15] # four corners
]
# returns True if a bingo card is a winner
def iswinning(matches):
for idx in win_idx:
if all([matches[i] for i in idx]):
return True
return False
# compute the probability that 5th card wins
# given that the 4th card did not win
N = 10**7
count,total = 0,0
n = 54 # number of cards
S = list(range(n)) # full set of numbers
C = random.sample(S,16) # 16 chosen numbers
for iter in range(N):
F = random.sample(S,5) # five drawn numbers
# bingo score card after 4 and 5 turns
matches4 = [c in F[:-1] for c in C]
matches5 = [c in F for c in C]
if not iswinning(matches4):
total += 1
if iswinning(matches5):
count += 1
# empirical probability and 95% confidence interval
phat = count/total
conf = 1.96*np.sqrt(phat*(1-phat)/total)
print(f'Analytical: {1728 / (-432 - 6*n + 11*n**2 - 6*n**3 + n**4):.5g}')
print(f'Monte Carlo: {count}/{total} = {phat:.4g}, or [{phat-conf:.4g},{phat+conf:.4g}] (95% CI)')

The result of running this code is:

Analytical: 0.00022768
Monte Carlo: 2292/9999478 = 0.0002292, or [0.0002198,0.0002386] (95% CI)

Unfortunately, since we are trying to estimate such a small probability, a large number of samples are required to reach a narrow confidence interval. I used 10 million samples in the simulation above, which took about 4 minutes on my laptop.