you held the 4000 shares for 6 months so you are entitled to dividends on those
nothing more, nothing less
you bought the others just before the ex div, so you arent entitled to dividends on those
the move you make by buying another 4k then selling the other 4k does very little. it also remove your 6 months of accumulated capital gains tax discount so you wont be entitled to that unless you wait another year
