The general term of the sequence is given as:

$t_n = \sum_{i=1}^{n} i$

This means that for each $n$, $t_n$ is the sum of the first $n$ natural numbers.

To find the first four terms of the sequence, we'll calculate:

1. $t_1 = \sum_{i=1}^{1} i = 1$
2. $t_2 = \sum_{i=1}^{2} i = 1 + 2 = 3$
3. $t_3 = \sum_{i=1}^{3} i = 1 + 2 + 3 = 6$
4. $t_4 = \sum_{i=1}^{4} i = 1 + 2 + 3 + 4 = 10$

So, the first four terms of the sequence are:

$t_1 = 1, \quad t_2 = 3, \quad t_3 = 6, \quad t_4 = 10$

Would you like further clarification on this or more details? Here are some related questions to explore:

1. What is the general formula for the sum of the first $n$ natural numbers?
2. How does this sequence relate to triangular numbers?
3. Can you express the $n$-th term using a closed-form formula instead of a summation?
4. What is the 10th term of the sequence?
5. How can this type of sequence be represented graphically?

Tip: The sum of the first $n$ natural numbers can be found using the formula $\frac{n(n+1)}{2}$.