The card is okay but not incredible. 4/2 in the 3 slot isn't great, there's already plenty of minions with that stat line that never see play. (Jungle Panther even has Stealth, and is a beast, and isn't played!)
+1/+1 on minions in hand hasn't proven to be very powerful as we've seen already with The Mistcaller which also has poor stats for it's mana slot.
This being limited to Deathrattle minions will suck for Standard decks, might see play in Wild.
Doing a card search, there's only 22 known deathrattle cards in Standard for Hunter, with maybe a few more yet to be released.
Notably:
Tentacle of N'zoth
Leper Gnome
Loot Hoarder
Huge Toad
Thalnos
Spawn of N'zoth
Harvest Golem
Twilight Summoner
Polluted Hoarder
Infested Tauren
Sylvanas
Highmane
Cairne
Chillmaw
Deathwing, Dragonlord
MAYBE there's some super secret N'zoth deck that will make it for Hunter, but I doubt it. I like N'Zoth but for him to be worth you want the Deathrattle minions to be huge, you don't want tiny shit minions reviving and Forlorn Stalker doesn't work as well with big minions, you want him on flood cards to get +1/+1 to lots of tiny shits for maximum value.
So yea, not seeing Forlorn Stalker being very good. I do think Hunter will have decent Deathrattle decks, but that's because of Infest which won't work with Forlorn Stalker.
Edit: 22 Deathrattlers