I don't think they'll be a fire hazard. My own tank heated up because of the light hitting the rocks, causing the water to warm. MH lights can be a fire hazard because the bulbs burn so hot. I think the heat from T5's affecting a tank is from the intense light warming the rocks.
But if the reflectors are separate, they clip onto the bulb. It's the sockets that get mounted to your hood if it's not a retrofit system.
4" should be fine if you have good fans circulating the air in the hood. I run my own 6" away from the water, hung from the ceiling so I can get my arms under easily to do maintenance. I have the believe if daily maintenance is a bother, it won't be done daily
