Where i could find GTA IV Product key in steam?

[Yelloface]

It was easy with an older steam design but i can't find it in a new one.

[Yujah]

That key doesn't exist anymore in the context of the current "GTA IV: The Complete Edition" version (in fact, Microsoft stopping key generation was the very reason for said TCE). If you want it for GFWL-multiplayer on a downgraded copy, https://gtaconnected.com/ may be a better option.

If you want it for recovering a Rockstar account with a copy of GTA4 bought from Steam back when it wasn't yet upgraded to the TCE, Steam support should be able to supply you with it If you go through Support -> <gta4> -> "I have a question about this product". Be sure to explain you need the key from when you bought it due to Rockstar needing it for Rockstar account recovery.

[Yujah]

Note; verified that Steam support will/can not in fact (anymore) supply you with a GFWL-key for a once bought GFWL-version of GTA4 and/or EFLC.

[Bradly105]

GTA 4: Complete Edition no longer provides keys due to the removal of GFWL from this version of the game. If you have a Steam game that requires a CD key, you can find it by right clicking the game in your library, hovering over manage and clicking on CD Keys.
If you need a key for GFWL, here are a few games that still provide keys:
Virtua Tennis 4
The Club
STAR WARS The Clone Wars Republic Heroes
Mortal Kombat: Arcade Kollection