90-min before departure should be sufficient for a 1K. If you're staying the last night in BRU, I recommend staying in Brussels City Centre instead of near the airport -- it's much easier/cheaper to get to the airport from the centre by train than from the hotels (by taxi), as most hotel shuttles do not run that early and taxi will easily run you 20-25€ for the short ride to the airport.
So it should be open at 3h15 but since it's only a intra EU flight chances excist that the checkin will only be open at 4h15.
This is an intra-EU flight, BUT NOT an intra-schengen flight, so passport control is required.
The lounge in BRU is nothing to write home about. Showers are offered at the lounge next door, good coffee, croissants and other pastries. Soup at lunchtime and a moderate selection of wine. Computers with Belgian keyboards (
) also exist, as does wifi. The BMI lounge (which closed at the end of Feb) was comparable.
For intra-schengen flights, I'd recommend an extra 30-min due to the long walk that to the the schengen terminal.