OK weather/meteorology enthusiast types. Can a hurricane decrease in velocity to a cat 4 and then go back up to a cat 5 before it hits land? Would this be unusual?

That is all.

From what I've read, the probability is low.  The hurricanes pick up energy in the warm, deep water in the middle of the Gulf, but as the water shallows out near the coastline it doesn't have that energy well to draw from, so it starts to lose rather than gain energy.

I could swear they could go up and down if the conditions were right.

Quoted: From what I've read, the probability is low.  The hurricanes pick up energy in the warm, deep water in the middle of the Gulf, but as the water shallows out near the coastline it doesn't have that energy well to draw from, so it starts to lose rather than gain energy.

Yes, they can go up and down.  However, limaxray is correct that in order for a hurricane to increase in strength it needs a source for that energy, such as warm water.   Similar to Katrina, as they approach shallower and/or cooler water they will typically loose strength.  

yeah, rita is still pretty far out. it can still spike.

Looks unlikely, right now experts are saying that it appears Rita is in an eyewall replacement cycle, meaning it weakens while it strengthens the eyewall. When the eyewall is done strengthening, it is able to grow again, but by then it will be over slightly shallower, and cooler waters, which will not provide fuel for it to get stronger than it is. Most feel that it may strengthen a little bit after eyewall replacement cycle, but it just depends on how much of that it will lose when it approaches the shore and weakens over the shallower/cooler waters......but water even close to shore is still VERY warm, its not like this thing is going to weaken to a cat 1/2, it will still be a monster......