B2 layed her 4th egg today but her 3rd egg was laid on Sunday so i was suprised to see a egg. i thought that eggs where laid 2 days apart so was i wrong in assuming that.

No, not wrong. But budgies are full of surprises :hap:

It was a big surprise, but since sunday she has been in the nestbox and has not been letting the male near her so would the egg be fertile.

The hen I have nesting now always seems to lay the last one three days apart! Maybe sometimes it just takes a while to work up to the next one, I take it as a sign that she is running out of egg laying steam and that she needs a rest from laying after that clutch.

Could she perhaps have been eggbound for those two days then managed to pass the egg?

She never showed any signs of being eggbounded, normal poos and very relaxed in her nestbox. Maybe like MB's hen she was probably having a rest.

- Could also have been that there was delays in the mating too ......

Regardless of the reason... Congratulations on egg number 4!!!