As a player, you may only enter 4 horses in any one event. It doesn't matter if some of the horses are leased and you return them to the owner prior to the event running. It doesn't matter if you enter them from more than one account. Once you enter 4 horses, you should not enter any more horses. You can be eventing as many horses as you desire, but can only have a maximum of 4 horses in any single event.
Leased horses are counted two ways, both the player that is eventing them and the player that owns them. If you lease horses out and other players enter more than 4 of your horses in an event, you can get into trouble.
The only exception to this are train-off events from train-offs posted in the train-off section of the forums. Train-off events are given an exception because they promote the community.
Added from the original rules: Trainoff horses may enter non trainoff events if the train off host allows it. However, these would still be non trainoff events and would follow the four horses rules like any other non trainoff event. Having six or more participants does not circumvent the four horse rules in public events.
Train off rules can be found here: viewtopic.php?f=27&t=137653
NOTE: The four horses per player rule does apply for non public events also when there are less than 6 active players competing in the event. Yes, that means if you only have 5 people in your train off, no matter the number of horses involved, if you (the host) enter fillers it will count against the four horse rule and result in suspension. (Unless approved by an admin before hand, for instances such as players drop out and you're wanting to run the last events to end the train off.)
If you purchase horses it is your responsibility to also ensure they are not in the events you already have four horses in. Not doing so will result you in going over your four horse limit and risking suspension. There is no way for us to know the horse was purchased validly or if it was an attempt to get around the rules.
If you share an account with someone, you must coordinate your entrances to avoid having more than 4 horse entered into an event. When you give someone your login information, you are considered sharing the account and you both own it. When we see that an account is owned by a player and there are more than 4 horses entered from any related accounts, we consider the same player is entering all the horses. We have no way of knowing other wise. This includes accounts you may have leased to other players, as they are still considered your accounts.
If there is more than 1 person playing from the same IP, we have to assume that the same player is entering the horses. So you need to talk with other players in your house who enter events to make sure you do not enter more than 4 horses in any single event between you. This is just like when an IP shows more than the allowed accounts being accessed in game. Everything on one IP is considered owned and accessed by the same person and the people on the IP must coordinate how many accounts they create to stay within the rules.