Technically, Money Heist season 4 is the second half of the second season, which is why Netflix refers to it officially as Money Heist: Part 4. Hence, it picks up right where the third season left off, with the Professor (Álvaro Morte) falling in the trap he set and kickstarting a war in the process, while the police officer Raquel `Lisbon` Murillo (Itziar Ituño) herself in handcuffs.
Here's the official synopsis for Money Heist season 4, via Netflix:
`La casa de papel: Part 4 begins in chaos: the Professor (Morte) thinks that Lisbon (Ituño) has been executed, Rio (Cortes) and Tokyo (Oliveira) have blown up an army tank, and Nairobi (Jimenez) is struggling between life and death. The gang is going through one of its toughest moments and the rise of an enemy among its ranks will put the heist in serious danger.`
All eight episodes of Money Heist season 4 are available on Netflix. You can also download them for offline viewing.