Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.

It is the future perfect.

It expresses that by a certain point in the future, the action will already be completed. In this sentence, the speaker means that four days from now, the report will be finished.

Compare:

• Θα τελειώσω την αναφορά. = I will finish the report.
• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά. = In four days, I will have finished the report.

The second one focuses on the completed result by that future time.

It is formed with:

• θα
• έχω (present tense of έχω, to have)
• τελειώσει (the form used in perfect constructions)

So the pattern is:

θα + έχω + perfect form

Examples:

• θα έχω γράψει = I will have written
• θα έχω δει = I will have seen
• θα έχω φύγει = I will have left

In this sentence:

• θα = future marker
• έχω = I have
• τελειώσει = finished

Together: I will have finished

Here σε means in, in the sense of after a period of time / from now.

So:

• σε μία ώρα = in one hour
• σε δύο μέρες = in two days
• σε τέσσερις μέρες = in four days

This is a very common Greek way to express time from now until a future point.

Do not confuse this with duration:

• σε τέσσερις μέρες = in four days / four days from now
• για τέσσερις μέρες = for four days

Because μέρες is a feminine plural noun, and the numeral agrees with it in the form commonly used here.

• singular: η μέρα
• plural: οι μέρες

So:

• τέσσερις μέρες = four days

You will often see:

• τέσσερις γυναίκες = four women
• τέσσερις ώρες = four hours

With neuter nouns, you get:

• τέσσερα βιβλία = four books

Both exist.

• ημέρα / ημέρες is the more formal or full form
• μέρα / μέρες is the very common everyday form

So these are both correct:

• σε τέσσερις μέρες
• σε τέσσερις ημέρες

In normal spoken Greek, μέρες is much more natural.

Greek uses the definite article much more often than English.

Here την αναφορά means the report, referring to a specific report that speaker and listener already know about.

• η αναφορά = the report (nominative)
• την αναφορά = the report (accusative, used here as the direct object)

Since τελειώνω takes a direct object, αναφορά appears in the accusative:

• τελειώνω την αναφορά = I finish the report

Because the noun is the direct object of the verb.

The basic dictionary form is:

• η αναφορά = the report (subject form / nominative)

But after a verb like τελειώνω when it receives the action, Greek uses the accusative:

• τελειώνω την αναφορά = I finish the report

So:

• η αναφορά είναι έτοιμη = the report is ready
• τελειώνω την αναφορά = I am finishing the report

This is a very important difference.

Θα τελειώσω την αναφορά

This simply means I will finish the report. It states a future action.

Θα έχω τελειώσει την αναφορά

This means I will have finished the report. It emphasizes that by some future moment, the action will already be completed.

So:

• Αύριο θα τελειώσω την αναφορά. = Tomorrow I will finish the report.
• Μέχρι αύριο θα έχω τελειώσει την αναφορά. = By tomorrow I will have finished the report.

In your sentence, σε τέσσερις μέρες gives that future reference point.

Yes. Greek word order is flexible.

These are all possible:

• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.
• Θα έχω τελειώσει την αναφορά σε τέσσερις μέρες.
• Την αναφορά θα έχω τελειώσει σε τέσσερις μέρες.
  (more marked, with emphasis)

The most neutral version here is probably:

• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.

Putting the time phrase first often helps set the time frame immediately.

Not exactly, even though it may look like one to a learner.

In θα έχω τελειώσει, the word τελειώσει is the form used with έχω to build perfect tenses.

You will see the same form in:

• έχω τελειώσει = I have finished
• είχα τελειώσει = I had finished
• θα έχω τελειώσει = I will have finished

So it is not functioning by itself here as a simple past tense. It is part of a compound tense.

Yes, but the nuance is slightly different.

• σε τέσσερις μέρες usually means in four days / four days from now
• μέσα σε τέσσερις μέρες often emphasizes within four days

So:

• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.
  = In four days I will have finished the report.

• Μέσα σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.
  = I will have finished the report within four days.

The second one can stress the time limit or maximum time needed.

Yes, it is natural and correct.

It sounds like something someone might say when talking about a deadline, a plan, or expected progress.

For example:

• Μην ανησυχείς. Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.
• We don’t need to worry. In four days I’ll have finished the report.

The future perfect is not used in every situation, but it is perfectly normal when you want to stress completion before a future point.

You put δεν before the verbal part:

• Σε τέσσερις μέρες δεν θα έχω τελειώσει την αναφορά.

This means:

• In four days, I will not have finished the report.

Greek normally uses:

• δεν θα έχω τελειώσει

not something like moving the negative later in the phrase.

Greek usually makes yes/no questions with intonation, not by changing the word order.

So you can simply say:

• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά;

This means:

• Will I have finished the report in four days? or, depending on context,
• Will you have finished the report in four days?
  if the subject is understood differently in another sentence

In writing, Greek often uses the semicolon (;) as the question mark:

• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά;

No. Greek usually leaves subject pronouns out unless they are needed for emphasis or contrast.

So the normal sentence is:

• Σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.

If you say:

• Εγώ σε τέσσερις μέρες θα έχω τελειώσει την αναφορά.

that gives extra emphasis, something like:

• I, for my part, will have finished the report in four days.

Greek verbs already show the person, so έχω tells you the subject is I.