Hey-
Just for my part, you asked what's realistic and from following various other low carbers in the diary section here and in other low carb forums, around 2lbs per week is the most common loss. Obviously it's different for everyone - some lose more, some less, my own loss seems to bounce around from 3lbs one week to 1lb the next for no real rhyme nor reason - but it does even out to about 2lbs per week, so far.
Others closer to their goal seem to lose slower and those with more to lose can lose more. Again that's not set in stone, just the most common thing I've seen.
I have also done VLCD and yes, probably lost more like 3-4lbs a week you're absolutely right. But low cal and low carb diets involving eating 'real' food rarely cause quite such speedy loss. That's why people are willing to do the total meal replacement diets regardless of how hard they can be - for the speed. It's why I did them, anyway.
So for me, and from my own observations, 2lbs a week is a realistic loss. 3.8lbs a week would be a great loss. There's absolutely no harm in shooting for the higher figure - it's not something I tend to do myself just because if I don't meet goals I feel like a failure and it can lead to my slipping off the wagon. If it will motivate you to stay on track - go for it!
I'm sure all the exercise you do also has positive effects.
Whatever happens you'll be fitter and healthier for keeping on your plan and that can only be good. No one here wants you to fail - or anyone to. I think all of us are familiar with the frustration of weight loss being less speedy than we'd like - personally I always feel like the weight should just drop off when I'm being virtuous. If only our bodies would comply.
Good luck, sounds like you're driven xx