The 4 washers is equal to about a degree of correction if I remember correctly. I was dropped 2" also and the 4 washers was not enough. After three months of riding around with the bolts and washers on the rear and nothing on the front my rear tires were perfect and my front were bald on the inside. You don't necessarily need a longer bolt to use more washers. Is is safe to use more washers? Reasonably safe, as long as you aren't putting 8 washers on or something like that. I wouldn't go past 6, but that is just me.