We can take integers as $k$ bits in the system. For an easy explanation, we are taking $k =4$.
$s1 = 1001
s3 = 0010$
$u1 = 1010
u3 = 0001$
$s3$ and $u3$ will be $2$ and $1$ in decimal, respectively, irrespective we treat them as signed numbers or unsigned numbers (MSb is $0$).
$s1$ is $9$ in unsigned and $-7$ in signed.
$u1$ is $10$ in unsigned and $-6$ in signed.
This is an unsigned comparison i.e., $s3$ will be treated as an unsigned number.
Although MSB of $s3$ is $0$ hence, it does not matter.
$s3 > u3$ is same as $2 > 1$, which is true
both are signed hence signed comparison
$s1 > s3$ is same as $-7 > 2$, which is false.
both are unsigned hence unsigned comparison
$u1 > u3$ is same as $10 > 1$, which is true.
one of the variables is unsigned hence unsigned comparison
$s1 > u3$ is same as $9 > 1$, which is true.