Ultimately, the only way truly to know something is by personal experience. After I experienced the Blue Heart Event of the eligible girls in Harvest Moon Boy & Girl, I raised them to yellow heart level and then waited in vain to experience another Heart Event... Gradually, their heart levels climbed higher. When Ann reached orange heart level, I decided to test a theory. I purchased the Blue Feather from the Supermarket, raised her heart level to deepest red, then proposed to her. (I had no intention of saving the result, by the way. I still need to experience the Starry Night Festival with each girl.) Ann rose faster to red heart level probably because of the extra events she has, such as her Birthday Party Event, rather than any personal preference on my part.
In any event, she responded to my proposal with the words: Really? Are you sure you want a girl like me? You won't regret it later? ...I... I'm happy... but...
What was that 'but...'? Whatever it signified, she had taken the Blue Feather and, a week later, our wedding took place at the Church. In time-honoured Harvest Moon fashion, then, Ann asked me to choose a nickname and our married life would have commenced, except that I reloaded my game back to the eve of the Starry Night Festival, single once again.
No need now to waste any more time hitting all the likely spots for a fourth Heart Event for any of the girls. I do believe, however, that there are four rival heart events and, when I decide if I wish to marry any girl and which one I intend to choose, I can lower the heart levels of the others to experience the two remaining rival heart events and marriage of the other couples in this game.
Incidentally, in HM Boy & Girl, you can raise either any eligible girl or bachelor a full heart level or more in a single day provided you have sufficient 'Most Favourite Items' to squander in this pursuit. Unlike HMDS and HM Cute DS, where only the first gift will raise AP, in Harvest Moon Boy & Girl, every item given in a single day will have an effect, although subsequent gifts have less value than the first item given. Now that I know there is no fourth heart event, I can propose to all the eligible girls, apart from Karen, successfully before the end of the first year if I wish. The only reason one cannot marry Karen in the first year is because her Third Heart Event cannot be experienced before Spring of the 2nd year. (Perhaps I should see if I can marry her without ever experiencing that event...) It therefore very much IS possible to marry a girl in HM Boy & Girl before the end of the first year.