Step 1: Let coordinates of \(P\) and \(Q\).
Let
\[
P=(x_1,y_1)
\]
and
\[
Q=(x_2,y_2)
\]
Since midpoint is \((1,5)\),
\[
\frac{x_1+x_2}{2}=1
\]
and
\[
\frac{y_1+y_2}{2}=5
\]
So,
\[
x_1+x_2=2
\]
and
\[
y_1+y_2=10
\]
Step 2: Express \(Q\) in terms of \(P\).
\[
x_2=2-x_1
\]
and
\[
y_2=10-y_1
\]
Step 3: Use line equations.
Since \(P\) lies on
\[
3x+4y-4=0,
\]
we get
\[
3x_1+4y_1=4
\]
Since \(Q\) lies on
\[
5x-y-4=0,
\]
we get
\[
5x_2-y_2=4
\]
Substitute \(x_2=2-x_1\) and \(y_2=10-y_1\):
\[
5(2-x_1)-(10-y_1)=4
\]
\[
10-5x_1-10+y_1=4
\]
\[
y_1-5x_1=4
\]
Step 4: Solve for \(P\).
We have
\[
3x_1+4y_1=4
\]
and
\[
y_1-5x_1=4
\]
From second equation,
\[
y_1=5x_1+4
\]
Substitute in first equation:
\[
3x_1+4(5x_1+4)=4
\]
\[
3x_1+20x_1+16=4
\]
\[
23x_1=-12
\]
\[
x_1=-\frac{12}{23}
\]
Then,
\[
y_1=5\left(-\frac{12}{23}\right)+4
\]
\[
y_1=\frac{32}{23}
\]
Step 5: Find \(Q\).
\[
x_2=2-x_1=2+\frac{12}{23}
\]
\[
x_2=\frac{58}{23}
\]
\[
y_2=10-y_1=10-\frac{32}{23}
\]
\[
y_2=\frac{198}{23}
\]
Step 6: Find slope of \(PQ\).
\[
m=\frac{y_2-y_1}{x_2-x_1}
\]
\[
m=
\frac{\frac{198}{23}-\frac{32}{23}}{\frac{58}{23}+\frac{12}{23}}
\]
\[
m=\frac{\frac{166}{23}}{\frac{70}{23}}
\]
\[
m=\frac{166}{70}
\]
\[
m=\frac{83}{35}
\]
Therefore,
\[
\boxed{\frac{83}{35}}
\]