The Way of the Sun Soul monk (Xanathar's Guide to Everything, p. 35) gets the Radiant Sun Bolt feature at 3rd level:

You gain a new attack option that you can use with the Attack action.
This special attack is a ranged spell attack with a range of 30
feet.

They also get the Searing Arc Strike feature at 6th level:

Immediately after you take the Attack action on your turn, you can
spend 2 ki points to cast the burning hands spell as a bonus action.

The Spell Sniper feat (PHB, p. 170) has the following prerequisite:

Prerequisite: The ability to cast at least one spell

I was recently designing a Shadar-kai Monk who would take the Way of the Sun Soul archetype (ironic, I know). In the process, I began thinking about how he would benefit enormously from the Spell Sniper feat (increase his range from 30 to 60 ft). I then noticed the prerequisite on the feat and began thinking...

Does a ranged spell attack count as casting a spell (thus allowing the Sun Soul to pick up Spell Sniper at level 4)?

Or does the Sun Soul have to wait until he gains Searing Arc Strike at level 6 to qualify for Spell Sniper for his level 8 feat?

The Sun Soul will have to wait until level 6 to qualify for Spell Sniper

A ranged spell attack is not the same as casting a spell. Several different sources could give a character the ability to use a ranged spell attack, but unless the feature says "cast [some spell]" it doesn't count.

Searing Arc Strike on the other hand explicitly says the monk casts the spell, so it makes the character qualify for the the Spell Sniper feat.

Note that Spell Sniper wouldn't enhance the range of Radiant Sun Bolt, because it's not a spell. It would make RSB ignore half and 3/4 cover, since that applies to all ranged spell attacks

Not as soon as you hope, and not as well as you hope.

Any character who can cast a spell by any means qualifies for Spell Sniper. The ability to do so via Ki points still qualifies, you just have to wait until you actually have the ability (L6 in this case).

However, Radiant Sun Bolt's range will not benefit from Spell Sniper. That specific benefit requires "a spell that requires you to make an attack roll". Radiant Sun Bolt is very specifically still the Attack action - it is not a spell.

Radiant Sun Bolt will benefit from the cover mitigation, because that only affects "your ranged spell attacks".

There are other feats that work the same way. Crossbow Expert allows the character to avoid disadvantage for any ranged attack roll within 5 feet. It isn't restricted to just crossbows, despite the name of the feat.