+4 Level Of All Projectile Skills- how powerful is this thing?

Just upgraded a normal crossbow into a Sorrow Core Taut Crossbow. I’m level 64, Act 3 Cruel, and this particular modifier seems fairly darn strong when compared to many of the other modifiers I’ve been seeing in my 100 hours with the game. I’m using a crossbow and this has increased 90% of my skills’ levels by around 30%. Is this a rare modifier to see or is it fairly commonplace?

it's a good mod but doesn't mean much on a bad weapon

Understood- the weapon is good also and so I’m definitely very happy with this development. My Hailstorm bullets went from Level 14 to Level 19 as I whacked an additional +1 Level support gem on it. You can play for 30 hours without a meaningful upgrade and these rare moments of discovery are awesome.