I normally go to sleep at 11 and I usually find myself waking up at around 4am, which is about 4 hours after going to sleep. so does that mean I wake up before/during or after REM? will it have any effects on WBTB?
I have been trying to combine WBTB and WILD together, if I wake up after REM, does that mean I have to remain conscious for the next 90-130 mins until the next REM? then how does it make any differences from WILD-ing before going to sleep?
I recognize my quote here 
so I feel like I should give you some kind of answer…
Sleeping cycles repeat over and over. I will not go in depth on sleeping cycles here, you have wikipedia for further study which I would encourage you to do so because it’s the base of understanding dreams and lucid dreams.
The main logic behind it is that cycles before REM become shorter and REM becomes longer so in that cycle of 90 - 130 minutes (which is and average number and not may apply to you at all) you will enter REM every time at different time depending on how much you have slept before entering one specific REM cycle.
Without being wired to a machine you don’t have an exact way to determine when your REM cycle occurs, it all comes down to a feeling. Which I explain in other topic from which this quote was. Again it may or may not apply to you it’s very subjective.
so that means the later I wake up the better? guess I will try setting an alarm after 6hours then. also I have seen some people in the forum said that its better to go back to sleep right after you wake up, is that true? normally I would stay awake for 45 mins, which hasnt been very effective so far (just some more vivid NDs and FAs). say if I want to perform WILD with WBTB, how long should I stay up for?
again I know it depends on people, just want to hear some general ideas 
Yes the longer you sleep the longer your REM cycle becomes which means more dream time once you fall asleep.
Just because you didn’t succeed at technique it doesn’t mean you weren’t effective. You have to appreciate any progress you make, it’s still better then no dream recall at all, isn’t it? That’s a great thing when it comes to WILD. Even do you don’t achieve WILD you are still having a possibility to either become lucid from a non lucid dream later or from a FA because you did raise your awareness in the middle of your sleep by doing WBTB.
When it comes to WILD only think you need to know is how to fall asleep properly. Not to fast not to slow. Just at the right speed so to say that’s why it’s important to know for yourself how long to stay awake after WBTB.