UPDATE: Now that Warriors Orochi 3 is available on Steam, I would heartily recommend WO3 instead of WO4 - it's got loads more contents (I allude to some of 4's critical missing content in my original review below, and the Steam version of 3 comes with all of its DLC as part of the base purchase), is much cheaper, and actually has a story that's halfway interesting.

Original review:

There is no denying that Warriors Orochi 4 was hastily thrown together as sort of an "apology game" to the spectacular dumpster fire that was Dynasty Warriors 9 - as even by Koei's standard the amount of effort on display is questionable. Leaving that aside, this game can pretty much be described as "just another Warriors game". The following comments will be split into the "not a Warriors fan" section and the "hardcore Warriors fan" section:

If you're not a Warriors fan:
There's nothing here that may change your mind if you're already committed to thinking that Warriors games are just endless button mash with slightly exotic historical/mythological characters using strange weapons, and reading further is likely going to be a waste of your time.
However, if you do dabble in hack 'n slash now and then, you'll find a perfectly functional Warriors game here, with the mechanics of the Orochi series (3-character tag team), with the new combat mechanics that was introduced to Samurai Warriors since the last Orochi game added in (hyper attacks, which I personally quite enjoy), plus a bit of extra sprinkled on top (the new magic attacks, but functionally they're just a new array of attacks that rather generic). You'll find the new characters (Greek and Norse gods) to be decently fun to play, but the new story somehow even more lazily written than the previous Orochi games.
All in all, a decent buy if it's on sale, but otherwise you may feel a bit underwhelmed by the content on display (even considering the ridiculous number of playable characters).

If you're a hardcore Warriors fan:
Other than the fact that the PC version actually runs well, this game is strictly inferior to WO3U in basically every way. None of the changes contribute to making the game better, these include:

• New story - totally phoned-in and pointless, somehow they managed to make a clash of multiple pantheons boring
  
• New Greek & Norse characters - fun to play, but too few by Orochi game standard
  
• Old movesets - no moveset update of any kind. Zero, zilch, zip, nada, nothing. Keep in mind these are DW8 characters, who are over 5 years old, and old WO characters, who range from over 10 years old (god it's been that long?) to about 7 years old. Even the SW4 characters are almost 5 years old at this point...
  
• Hyper attacks - only the Samurai Warriors characters get them (exactly as they were in the SW4 games), and after playing with the exact same ones for 3 whole games (SW4, SW4-II, and Sanada - I don't usually play Empire titles), they're kind of getting boring... (And no, they don't even have the 5 new Spirit of Sanada characters in WO4)
  
• Magic system - just a new array of attacks that do very little to add to the combat
  
• Removal of other KT characters - that's right, no DoA, no Ninja Gaiden, no Jeanne, no Achilles... Seriously? They were in WO3U ffs! What about the characters that already have movesets made for them for All-Stars? Like William? Just how lazy were these guys...

However, as a replacement for DW9, it serves its purpose and at least made me calm down enough to want to at least want to pay attention to the series again. But then again, DW6 can be considered an uncontroversial masterpiece compared to DW9, so maybe that's too low a bar to clear.
One last thing I want to mention for the Warriors fans (that I strangely, but not unexpectedly, appreciated in this game): finally a Warriors game that streamlined the mount to such a point as to be actually useful - there's now a dedicated button combination to summon your mount on the spot and instantly mount, without the need to faff about holding the whistle button or jumping instead of mounting or worrying about you're running to fast for your horse to catch up. Now this needs to be in all future Warriors games somehow...

This was the game that revitalized the DW series after the mixed bag that was 6 (or 5, if you're in non-English regions) - I personally liked it enough that I wouldn't mind it getting a Steam release too. While rereleasing a complete edition of a 2011 game on Steam now later feels like a diversionary tactic Koei is pulling to distract as much from the dumpster fire that is DW9 as possible, there's not denying that DW7 is still among the best entries of the series (Especially with the Steam version of 8 being rather unfortunately poor).

This was the first (and arguably only) DW game that strung the stories of each kingdom together in a coherent manner, having sacrificed character selection in the story mode (you are locked into a single character - not even pick from 3 like DW8 - for every story battle, thankfully pre-leveled appropriately if not already leveled), the story mode can focus on specific character interactions without breaking the flow or completely screwing with character positioning. This was also the first DW to introduce seamless cutscene-to-battle transitions - which I remember being quite enamoured by in 2011, but looks slightly janky now.

On the action front, after DW6's attempt at changing up the combat arguably fell flat on its head, Omega Force went back to the drawing board, and went with the classic light-strong attack stringing. It would be another 3 years and god knows how many spinoffs of various franchises, until Samurai Warriors 4 in 2014, before they decided to introduce another major mechanic (Hyper Attacks) into the action formula. Despite its age, the action system is definitely functional, and detaching characters and weapons definitely spice things up quite a bit - you can now equip your favorite characters with your favorite weapons, compatibility be damned!

Overall, for how much this game costs, you'll definitely get your money's worth, and going back to an actually good DW game will hopefully wash away some of that bad taste DW9 left in your mouth.