Je vois quatre photos.

Because voir is the infinitive, meaning to see.
In a normal sentence, French usually needs a conjugated verb, just like English needs I see, not I to see.

So:

• voir = to see
• je vois = I see

Here, vois is the 1st person singular form of voir in the present tense.

French verbs often change quite a bit when they are conjugated, and voir is an irregular verb.

Present tense of voir:

• je vois
• tu vois
• il / elle / on voit
• nous voyons
• vous voyez
• ils / elles voient

So vois is simply the correct present-tense form for je and tu.
It is not something you can predict from the infinitive in a perfectly regular way, so it is best learned as part of the verb’s pattern.

A simple pronunciation guide is:

zhuh vwah katr foe-toh

More carefully:

• je ≈ zhuh
• vois ≈ vwah
• quatre ≈ katr
• photos ≈ foe-toh

A rough IPA version is:

/ʒə vwa katʁ foto/

A few useful notes:

• j in je sounds like the s in measure
• oi in vois sounds like wah
• the final s in photos is silent
• in normal speech, je can sound very light, almost like zh'

Because after a number, French normally does not use an article.

So:

• quatre photos = four photos
• trois livres = three books
• deux chiens = two dogs

This works much like English: we say four photos, not the four photos unless we specifically mean those particular four photos.

If you wanted the four photos, you would say:

• les quatre photos

Because the number quatre tells us there is more than one photo, so the noun must be plural.

• singular: une photo
• plural: des photos or quatre photos

Most French nouns form the plural by adding -s, just like here:

• photo → photos

In pronunciation, that final s is usually silent, so photo and photos sound the same by themselves.

Because French numbers generally do not change form to agree with the noun.

So you say:

• quatre photo would be wrong
• quatre photos is correct

The noun changes to plural, but quatre stays the same.

Photo is a very common shortened form of photographie. It is completely normal and natural in everyday French.

So:

• une photo = a photo / a photograph
• une photographie = a photograph

Both are correct, but photo is far more common in ordinary conversation.

Usually, no important liaison is made in this sentence.

A few details:

• je vois: no liaison
• vois quatre: normally no liaison
• quatre photos: normally no liaison

So the words are mostly pronounced separately, though in fluent speech they flow together smoothly.

Because this sentence follows the standard French word order:

subject + verb + object

So:

• Je = subject
• vois = verb
• quatre photos = object

This is the same basic order as English I see four photos.

In informal spoken French, many speakers reduce je and it can sound like j’vois. That is very common in conversation.

But in standard written French, you should write:

• Je vois quatre photos.

So:

• je vois = correct standard writing
• j’vois = informal spoken-style spelling, not standard in normal writing

You put ne ... pas around the conjugated verb:

• Je ne vois pas quatre photos.

In everyday spoken French, the ne is often dropped:

• Je vois pas quatre photos.

But in standard written French, use ne ... pas.

Yes. In many cases, the French present tense can translate as either:

• I see
• I am seeing

So je vois can cover both, depending on context.

French does not use a separate form exactly like English I am seeing in most ordinary situations. The simple present often does the job by itself.

A very useful set to learn is the full present tense:

• je vois = I see
• tu vois = you see
• il / elle / on voit = he / she / one sees
• nous voyons = we see
• vous voyez = you see
• ils / elles voient = they see

It is especially helpful to notice:

• je vois and tu vois sound the same
• il voit sounds a little different
• ils voient is spelled differently, but in pronunciation it sounds like voient with the ending silent

Learning the whole pattern makes je vois easier to remember.