The Four Lovers

The Shitennou and the Senshi

Throughout the whole Bishoujo Senshi Sailor Moon and Sailor V manga series, there are hints that a romantic relationship existed between Prince Endymion's Four Generals and Princess Serenity's Four Guardian Soldiers. The most well-known proof of the Four Lovers is the picture used for the title page of the conclusion of the first arc of the manga series. This same picture is also printed in the first volume of the Sailor Moon Artbooks with the following note by Takeuchi Naoko:
This is the title page for the conclusion of the first series of Sailor Moon. It had a great deal of impact on the first series. Probably because the four couplings on the right side were very unexpected. I was thinking of love stories of the previous lives of these couples. I'd like to be able to draw that someday...1

This is why some hints of their relationships can be found throughout the manga. However, there are more hints of a romantic relationship existing between Minako and Kunzite than the other three couples. The first hint can be found in Codename wa: Sailor V, Volume 2, Chapter 8. In chapter 8, Minako develops a crush on Saitou, who greatly resembles Kunzite. Like Kunzite, Saitou has shoulder-length white hair and the same prominent facial features. Minako's musings on Saitou points out the obvious physical similarities between the two men:
Nice, Minako! [...] His white shirt is elegant. He's so cool! He's wearing a promise ring around his ankle. He looks like someone. But I can't remember. I wonder who it is... Maybe some idol I saw. Yeah, hmmm...2

Venus and Kunzite exchange glances This means the similarity in looks was intentional and serves as a clear hint that a special relationship might have once existed between the two. Especially, if Minako subconsciously remembers him. The next hint of their past relationship can be found in the Sailor V manga. Kaitou Ace mentions that even in the past, Venus fell in love easily and then we see his memory of Venus staring lovingly into Kunzite's eyes. Another hint can be found in the Sailor Moon manga. In Volume 3, Chapter 12, Sailor Moon reflects on the past and remembers one failed attempt at sneaking down to Earth. Venus scolds Serenity, telling her it is Venus and Kunzite talk dangerous to meet Prince Endymion just for fun. Princess Serenity retorts that she isn't doing it "just for fun" and says that Venus wouldn't understand because she has never fallen in love before. Venus mumbles to herself that Princess Serenity is mistaken and immediately thinks of a conversation she had with Kunzite. As for possible relationships between the other three lovers, there are not much hints of it. Jupiter and Mercury's relationship with Nephrite and Zoisite are never shown in the manga. As for Rei's relationship with Jadeite, there is only proof that he finds Rei extremely attractive:

So beautiful...I have wanted her ever since I first saw her. This girl...3   
This does not necessary mean that a romantic relationship did exist between the two during the Silver Millennium as the quote above is unclear whether Jadeite is referring to first time he saw her in the present or in the past. It is highly likely that he was referring to the first time he saw her in the present as the Four Generals did not recover their memories of the past until after most of them have died. However, his admiration for Rei cannot be denied and could mean he might have admired her back then too.

Kunzite remembers

However, in the new reboot of the Sailor Moon anime, Sailor Moon Crystal, the romantic relationship between the four guardians and generals is now a confirmed fact. From Venus's first encounter with Kunzite till the last, she keeps begging him to remember who he really is. The reason for such a plea is finally revealed in Act 10 - Moon, when during a battle with the four generals, Venus finally tells the rest of team who the four generals really are, the four loyal knights of Prince Endymion. This revelation leads to Mars, Mercury, and Jupiter remembering that they had fallen in love with the generals in their past lives. Unfortunately, the four generals have become too corrupted by Queen Beryl to listen. Then in Act 12 - Enemy, during another battle between the four guardians and the four generals, the generals finally regain their memories after getting hit by a Sailor Planet Attack. Kunzite and Venus exchange a look of tenderness before Metalia kills the generals for their uselessness with a blast of negative energy. The four guardians openly mourn for deaths and after some comforting words and a gentle reminder of their duty from their former lovers, the four guardians continue on their way to save Sailor Moon from Metalia.