I've just primed 4.5L of weissbier with 38g of demerara sugar (the only I had around) and bottled it. According to beersmith's calculator that's what I should add to achieve 3.8 volumes of Co2. After some googling, I gather the values seem to vary a bit, so I'm slightly concerned. Could I be pushing it too much and risking bottle bombs? (I should add that Beersmith was initially giving a 2.3-2.9 for that type of beer but after some googling I've found their range was wrong and updated it to 3.3-4.5.)
It's "only" 8.4 g/L which is IMO is completely fine.
Looks like Beersmith has a more optimistic carbonation formula than most other calcuators.
I find brewers friend one to get close to my expected carbonation (subjectively at least, don't have the means to measure actual level). It gives about 5g/L for 2 volumes, and 8-10g/L for about 3 volumes for sucrose at ale temps.
Last time I brewed a Weissbier, I added ~8g/litre of white table sugar, which seems about the same as your case. My beer turned out somewhat overcarbonated and gushing out on opening when not chilled. However, I think it was far from producing bottle bombs.
As a preventive measure, you might want to make sure the bottles aren't sitting in a too high temperature during bottle conditioning. When you open the first bottle, you'll get a hint regarding the carbonation level. You might want to lift the bottle caps on the remaining bottles and let some of the CO2 out.