Explanation:

- Scientists at Oxford University are working on a coronavirus vaccine.

- They report it shows promise in terms of immune response in early trials.

- Early trials are primarily designed to look at safety concerns.

- Here, they also assess the kind of immune response participants have.

Now, let’s examine the options for filling in blank no. 4:

1. Elevate: It means to raise or lift up. This doesn’t fit the context.

2. Criticise: It means to express disapproval. This is unrelated to the trial’s purpose.

3. Dictate: It implies imposing control, which doesn't match the trial's objective.

4. Evaluate: It means to assess or appraise. Trials aim to evaluate safety and immune response.