Given real numbers $x,y,z,t\in (0,\pi /2]$ such that 
$$\cos^2 (x)+\cos^2 (y) +\cos^2 (z) +\cos^2 (t)=1.$$
What is the minimum possible value of $$\cot (x) +\cot (y) +\cot (z) +\cot (t)?$$
Given the problem, we need to find the minimum possible value of \(\cot(x) + \cot(y) + \cot(z) + \cot(t)\) under the constraint \(\cos^2(x) + \cos^2(y) + \cos^2(z) + \cos^2(t) = 1\) with \(x, y, z, t \in (0, \pi/2]\).

1. **Expressing the cotangent in terms of cosine and sine:**
   \[
   \cot(x) = \frac{\cos(x)}{\sin(x)}
   \]
   Similarly for \(y, z,\) and \(t\):
   \[
   \cot(y) = \frac{\cos(y)}{\sin(y)}, \quad \cot(z) = \frac{\cos(z)}{\sin(z)}, \quad \cot(t) = \frac{\cos(t)}{\sin(t)}
   \]

2. **Using the constraint:**
   \[
   \cos^2(x) + \cos^2(y) + \cos^2(z) + \cos^2(t) = 1
   \]

3. **Applying the Arithmetic Mean-Harmonic Mean (AM-HM) inequality:**
   The AM-HM inequality states that for non-negative real numbers \(a_1, a_2, \ldots, a_n\):
   \[
   \frac{a_1 + a_2 + \cdots + a_n}{n} \geq \frac{n}{\frac{1}{a_1} + \frac{1}{a_2} + \cdots + \frac{1}{a_n}}
   \]
   Here, we apply it to the cotangents:
   \[
   \frac{\cot(x) + \cot(y) + \cot(z) + \cot(t)}{4} \geq \frac{4}{\frac{1}{\cot(x)} + \frac{1}{\cot(y)} + \frac{1}{\cot(z)} + \frac{1}{\cot(t)}}
   \]

4. **Simplifying the harmonic mean:**
   Since \(\cot(x) = \frac{\cos(x)}{\sin(x)}\), we have:
   \[
   \frac{1}{\cot(x)} = \tan(x) = \frac{\sin(x)}{\cos(x)}
   \]
   Therefore:
   \[
   \frac{1}{\cot(x)} + \frac{1}{\cot(y)} + \frac{1}{\cot(z)} + \frac{1}{\cot(t)} = \tan(x) + \tan(y) + \tan(z) + \tan(t)
   \]

5. **Using the trigonometric identity:**
   We know that \(\sin^2(x) = 1 - \cos^2(x)\). Given the constraint, we can write:
   \[
   \sin^2(x) + \sin^2(y) + \sin^2(z) + \sin^2(t) = 4 - (\cos^2(x) + \cos^2(y) + \cos^2(z) + \cos^2(t)) = 3
   \]

6. **Applying the Cauchy-Schwarz inequality:**
   Using the Cauchy-Schwarz inequality in the form:
   \[
   (\sin^2(x) + \sin^2(y) + \sin^2(z) + \sin^2(t)) \left( \frac{1}{\sin^2(x)} + \frac{1}{\sin^2(y)} + \frac{1}{\sin^2(z)} + \frac{1}{\sin^2(t)} \right) \geq (1 + 1 + 1 + 1)^2 = 16
   \]
   Given \(\sin^2(x) + \sin^2(y) + \sin^2(z) + \sin^2(t) = 3\), we have:
   \[
   3 \left( \frac{1}{\sin^2(x)} + \frac{1}{\sin^2(y)} + \frac{1}{\sin^2(z)} + \frac{1}{\sin^2(t)} \right) \geq 16
   \]
   Thus:
   \[
   \frac{1}{\sin^2(x)} + \frac{1}{\sin^2(y)} + \frac{1}{\sin^2(z)} + \frac{1}{\sin^2(t)} \geq \frac{16}{3}
   \]

7. **Combining results:**
   Since \(\cot(x) = \frac{\cos(x)}{\sin(x)}\), we have:
   \[
   \cot^2(x) = \frac{\cos^2(x)}{\sin^2(x)}
   \]
   Therefore:
   \[
   \cot^2(x) + \cot^2(y) + \cot^2(z) + \cot^2(t) \geq \frac{16}{3}
   \]

8. **Finding the minimum value:**
   The minimum value of \(\cot(x) + \cot(y) + \cot(z) + \cot(t)\) occurs when each \(\cot(x)\) is equal, given the symmetry and the constraint. Thus, the minimum value is:
   \[
   \cot(x) + \cot(y) + \cot(z) + \cot(t) \geq 4 \sqrt{\frac{1}{4}} = 4
   \]

The final answer is \(\boxed{4}\).