In my sisters wedding!!!

You can accomplish a lot in 4 months. Break up your goals into small increments. Log EVERYTHING you eat and clamp down on a workout routine you can stick too. How much are you looking to lose?

Trying to lose more than 2 lbs a week is generally not a very healthy or reasonable goal.

Here is a handy chart on what to set as a goal based on how much you have to lose:
More than 75 lbs: 2 lbs/week
40-75 lbs: 1.5 lbs/week
10-40 lbs: 1 lb/week
Less than 10 lbs: 0.5 lb/week

Exercise will definitely help but, above all else, weigh everything that goes into your body. Eat foods that are nutrient dense but not calorie dense. Pick up an exercise that's something you enjoy and you will stick with. Set goals that are attainable and do them. Set your goal at either 1 lb / week or 1.5 lb / week and don't eat back your exercise calories. Don't go too low or you'll feel starved and won't do it. I actually have my goal at 1 lb / week but try not to eat my exercise calories (I do sometimes) and still lose about 1.5-2 lbs / week and feel full and satisfied most of the time. Twenty-five pounds is possible and you can definitely do it. Don't overdo yourself in the beginning because you'll burn out and do worse in the end. Remember, keep it sustainable. Weight loss requires dedication.

Are you picking out the dress or going somewhere where you have to order? Keep that in mind, too. Don't get ahead of yourself and order a size smaller. Alterations can be done! Protein will keep you fuller longer and fat will help with that, too. There's all sorts of options for both. If you do balanced meals with fat, protein and carbs you should be good to go.

Don't ignore strength training. It's more work but a lot of results. Your clothes will fit better and you will actually burn more calories at the same weight. Remember, too, when you're doing cardio, work at higher intensities. Steady state cardio where you're hardly even out of breath doesn't benefit you as much. Push yourself. Do intervals (alternating higher and lower intensity, e.g. walk for a minute, run for 5) to help out, too.

Co-sign on some sort of strength training! And variable intensity cardio.

And in my opinion, in 'deadline' type situations I think it's okay to approach your goals on the slightly more aggressive side of reasonable. (Does that make sense?) So assuming 4 months/12 weeks, 25 lbs would be ~2 lbs a week but I say go for it. Just realize that you will most likely want to re-evaluate after the wedding.