select random indices from 2d array

I want to generate a 2d random array and select some(m) random indices to alter their values by predefined values(m).

For an example here, I want to generate 4 by 4 matrix. Then select 4 random indices and alter their values with [105,110,115,120] this values.

random_matrix = np.random.randint(0,100,(4,4))

# array([[27, 20,  2,  8],
#        [43, 88, 14, 63],
#        [ 5, 55,  4, 72],
#        [59, 49, 84, 96]])

Now, I want to randomly select 4 indices and alter their values from predefined p_array = [105,110,115,120]

I try to generate all the indices like this:

[
    (i,j)
    for i in range(len(random_matrix)) 
    for j in range(len(random_matrix[i])) 
]

But how to select 4 random indices from this and alter their values from predefined p_matrix? I couldn't think of any solution because I have to ensure 4 unique random indices where I stuck badly, as randomness haven't that guarantee.

Can we generate random matrix and selecting indices in a single shot? I need that because if the size of m getting larger and larger than it will be getting slower (current implementation). I have to ensure performance also.