Determine the largest value the expression 
$$ \sum_{1\le i<j\le 4} \left( x_i+x_j \right)\sqrt{x_ix_j} $$
may achieve, as $ x_1,x_2,x_3,x_4 $ run through the non-negative real numbers, and add up to $ 1. $ Find also the specific values of this numbers that make the above sum achieve the asked maximum.
1. **Expression Analysis**:
   We need to determine the largest value of the expression 
   \[
   \sum_{1 \le i < j \le 4} \left( x_i + x_j \right) \sqrt{x_i x_j}
   \]
   where \( x_1, x_2, x_3, x_4 \) are non-negative real numbers that sum to 1, i.e., \( x_1 + x_2 + x_3 + x_4 = 1 \).

2. **Inequality Derivation**:
   Consider the inequality derived from the square of differences:
   \[
   \left( \sqrt{x_i} - \sqrt{x_j} \right)^4 \ge 0
   \]
   Expanding this, we get:
   \[
   x_i^2 + 6x_i x_j + x_j^2 \ge 4 \left( x_i + x_j \right) \sqrt{x_i x_j}
   \]
   Rearranging, we obtain:
   \[
   \left( x_i + x_j \right) \sqrt{x_i x_j} \le \frac{1}{4} \left( x_i^2 + 6x_i x_j + x_j^2 \right)
   \]

3. **Summing the Inequalities**:
   Summing over all pairs \( 1 \le i < j \le 4 \), we get:
   \[
   \sum_{1 \le i < j \le 4} \left( x_i + x_j \right) \sqrt{x_i x_j} \le \frac{1}{4} \sum_{1 \le i < j \le 4} \left( x_i^2 + 6x_i x_j + x_j^2 \right)
   \]

4. **Simplifying the Right-Hand Side**:
   Notice that each \( x_i^2 \) appears in the sum exactly 3 times (since there are 3 pairs involving \( x_i \)), and each \( x_i x_j \) appears exactly once for each pair:
   \[
   \sum_{1 \le i < j \le 4} \left( x_i^2 + 6x_i x_j + x_j^2 \right) = 3 \sum_{i=1}^4 x_i^2 + 6 \sum_{1 \le i < j \le 4} x_i x_j
   \]
   Since \( \sum_{i=1}^4 x_i = 1 \), we have:
   \[
   \left( \sum_{i=1}^4 x_i \right)^2 = \sum_{i=1}^4 x_i^2 + 2 \sum_{1 \le i < j \le 4} x_i x_j = 1
   \]
   Therefore:
   \[
   \sum_{1 \le i < j \le 4} x_i x_j = \frac{1 - \sum_{i=1}^4 x_i^2}{2}
   \]

5. **Combining Results**:
   Substituting back, we get:
   \[
   \sum_{1 \le i < j \le 4} \left( x_i + x_j \right) \sqrt{x_i x_j} \le \frac{1}{4} \left( 3 \sum_{i=1}^4 x_i^2 + 6 \sum_{1 \le i < j \le 4} x_i x_j \right)
   \]
   \[
   = \frac{1}{4} \left( 3 \sum_{i=1}^4 x_i^2 + 6 \cdot \frac{1 - \sum_{i=1}^4 x_i^2}{2} \right)
   \]
   \[
   = \frac{1}{4} \left( 3 \sum_{i=1}^4 x_i^2 + 3 - 3 \sum_{i=1}^4 x_i^2 \right)
   \]
   \[
   = \frac{1}{4} \cdot 3 = \frac{3}{4}
   \]

6. **Achieving the Maximum**:
   To achieve the maximum value, we need equality in the inequality. This happens when \( \left( \sqrt{x_i} - \sqrt{x_j} \right)^4 = 0 \), i.e., \( \sqrt{x_i} = \sqrt{x_j} \) for all \( i, j \). Thus, \( x_1 = x_2 = x_3 = x_4 = \frac{1}{4} \).

The final answer is \( \boxed{ \frac{3}{4} } \) with \( x_1 = x_2 = x_3 = x_4 = \frac{1}{4} \).