This is usually a flame free area so, for the most part, you wont have to worry about that. Regarding your question though, I think the general consensus will be that adding water is not something you should be doing. Having said that, there are no hard and fast rules in homebrewing so if adding water is producing good beer that you enjoy drinking then by all means, there are no rules here, just guidelines.
That leads me into the second part of your statement where you mention ending up with a smaller batch of beer that is stronger than what you predicted. For the most part, that 4.5 to 4 gallon batch will be the same strength as the five gallon batch, its not getting any stronger just because you lost some of it trub. Your inner mathematician must be thinking in terms of a balanced equation but it doesn't work that way.
Now to the fun part, it can work that way if you want. If you want to end up with five gallons of good beer that is the strength you planned on but don't have the room to ferment five gallons then I would consider starting off with five gallons that is stronger than you want, then when you lost some to trub you can then dilute with boiled water which will bring it back up to five gallons at the strength you want.
Having said all that, the above is just a suggestion if you absolutely must have five gallons. My advice would be to simply let it go and accept that not all the beer you make is going to make it out alive or simply buy a bigger carboy. I'm personally not a big fan of diluting already fermented beer.
Maybe others on here have a different or more informed opinion.
Hope this helps
wr
