All vectors in $\mathbb R^4$ orthogonal to two vectors Problem statement:
Find all vectors in $\mathbb R^4$ that are orthogonal to the two vectors
$u_1=(1,2,1,3)$ and $u_2=(2,5,1,4)$.
Progress:
a) Denote a vector $u_3=(v_1,v_2,v_3,v_4)$ My desire is to determine $u_3$ so that $\left \langle u_1,u_3 \right \rangle=\left \langle u_2,u_3 \right \rangle=0$
$\left \langle u_1,u_3 \right \rangle=(1,2,1,3)*(v_1,v_2,v_3,v_4)=v_1+2v_2+v_3+3v_4=0$
$\left \langle u_2,u_3 \right \rangle=(2,5,1,4)*(v_1,v_2,v_3,v_4)=2v_1+5v_2+v_3+4v_4=0$
Thus I end up with a system of equations I could not solve.
b) I know that the cross product of two vectors $a$ and $b$ results in a vector orthogonal to $a$ and $b$ that cannot be applied in $\mathbb R^4$.
 A: For a), you have the correct system of equations.  Do you know the Gauss-Jordan elimination algorithm?  When you apply it to your system of equations, you will find that there are two free variables, so you will have a two-dimensional subspace.
For b), you are correct that the cross-product is only defined for vectors in $\mathbb{R}^3$.
Intuitively, to be orthogonal to the given vectors $u_1$ and $u_2$ is equivalent to being orthogonal to the plane spanned by $u_1$ and $u_2$.  In $\mathbb{R}^4$, the set of all vectors orthogonal to a plane has dimension $4 - 2 = 2$, so it is also a plane.
SPOILER:  To check your work, the canonical basis obtained by solving your system of equations is $w_1 = (-3,1,1,0)$ and $w_2 = (-7,2,0,1)$.  I'd be happy to give more details if you're stuck, but it sounds like you are close to solving it on your own.
A: In the given system, you have a matrix
$
A=
  \begin{bmatrix}
    1 & 2 & 1 & 3 \\
    2 & 5 & 1 & 4
  \end{bmatrix}
$
and you need to find other two vectors in $\mathbb{R}^4$, so you can find the Null Space of $A$, as the Null Space $N(A)$ is orthogonal to Row Space $C(A^T)$, so all the vectors in the Null Space will be orthogonal to every vector in Row Space.
Step 1: Find $R = rref(A)$. 
\begin{equation}
R = rref(A) =
  \begin{bmatrix}
    1 & 0 & 3 & 7 \\
    0 & 1 & - 1 & -2
  \end{bmatrix}
\end{equation}
Step 2: Find $Rx=0$ where $x =
\begin{bmatrix}
    x_1 & x_2 & x_3 & x_4
  \end{bmatrix}^T$
Here $x_3$ and $x_4$ are free variables, so we need to solve the system
\begin{equation}
  \begin{bmatrix}
    1 & 0 & 3 & 7 \\
    0 & 1 & - 1 & -2
  \end{bmatrix}
  \begin{bmatrix}
    x_1 \\ x_2 \\ x_3 \\ x_4
  \end{bmatrix}=
  \begin{bmatrix}
    0 \\ 0 \\ 0 \\ 0
  \end{bmatrix}
\end{equation}
a) Putting $x_3=1$ and $x_4=0$, we get $x_1=3$ and $x_2=1$, so the first vector $x = \begin{bmatrix}
    -3 & 1 & 1 & 0
  \end{bmatrix}$
b) Putting $x_3=0$ and $x_4=1$, we get $x_1=-7$ and $x_2=2$, so the second vector $x = \begin{bmatrix}
    -7 & 2 & 0 & 1
  \end{bmatrix}$
Thus the complete system will be
\begin{equation}
B =
  \begin{bmatrix}
    1 & 0 & 3 & 7 \\
    0 & 1 & - 1 & -2 \\
    -3 & 1 & 1 & 0 \\
    -7 & 2 & 0 & 1
  \end{bmatrix}
\end{equation}
Here both the third and fourth rows are perpendicular to both first and second rows, this can be verified by taking dot products.
