Summation Notation Confusion

I am unclear about what the following summation means given that $\lambda_i: \forall i \in \{1,2,\ldots n\}$:

$\mu_{4:4} = \sum\limits_{i=1}^{4} \lambda_i + \mathop{\sum\sum}_{1\leq i_1 < i_2 \leq 4}(\lambda_{i_1} + \lambda_{i_2}) + \mathop{\sum\sum\sum}_{1\leq i_1 < i_2

I understand how this term expands:

$\sum\limits_{i=1}^{4} \lambda_i = \lambda_1 + \lambda_2 + \lambda_3 + \lambda_4$.

But, I don't understand what how this term expands

$\mathop{\sum\sum}_{\substack{1\leq i_1 < i_2 \leq 4}}(\lambda_{i_1} + \lambda_{i_2})$

Nor do I understand how this term expands

$\mathop{\sum\sum\sum}_{\substack{1\leq i_1 < i_2 $

Any help in these matters would be appreciated.