The Guardian Challenge reward is obviously based on the Patronus.
In the original HP lore, the Patronus Charm is described as an "an immensely complicated and extremely difficult spell" "regarded as advanced magic, far beyond N.E.W.T level", Remus Lupin himself said that it was "ridiculously advanced"
Yes, Harry learned it, but then, Harry had a particular strong talent for DADA. It is also mentioned in lore that very few witches/wizards can actually produce a fully corporeal patronus. (though this seems to be forgotten just a little by later books, and especially so in the movies iirc)
So it would make very little sense that a third or fourth year would normally be able to learn it.
Besides, the Dementors, or rather "Dementares" in the server are level 80, so there would be little point to getting the spell so early.
Not to mention in HP lore, you can't learn Apparation (aka, Appareo) until the witch/wizard was 17..ar a third or fourth year is only 13-14. On the server, you learn Appareo at level 51, in "6th year" iirc.
It honestly doesn't take that long to level up when you do quests etc. If everything was so easy as you would like it to be, this server would get boring pretty quick because you'd have done everything already. Just have some patience.
Besides, I find, like Cheddarsoup has said, that I don't go fighting dementares anyway, if anything, it's more of a cosmetic spell. The initial appearance of your patronus/guardian is a chicken, if you want another appearance you buy it in the store, so basically I'd say its a semi-cosmetic spell.