You are missing market return sequence and dividends.

2015 the SP500 had close to zero growth, but paid out 1.5% of the total price in profit to shareholders (dividends). Those total are re-invested when your stash.

When your growth happens and your returns are super important too.

If you have 100k and want to withdraw 4k per year, the market can do +7,+7,0,0,-3, -4, -7, +20 for a net change of +20 over , but your actual amount will be

• ( 100000 - 4000 ) * ( 1 + 7 ) = 102720
• ( 102720 - 4000 ) * ( 1 + 7 ) = 105630.4
• ( 105630.4 - 4000 ) * ( 1 + 0 ) = 101630.4
• ( 101630.4 - 4000 ) * ( 1 + 0 ) = 97630.4
• ( 97630.4 - 4000 ) * ( 1 + -3 ) = 90821.488
• ( 90821.488 - 4000 ) * ( 1 + -4 ) = 83348.62848
• ( 83348.62848 - 4000 ) * ( 1 + -7 ) = 73794.2244864
• ( 73794.2244864 - 4000 ) * ( 1 + 20 ) = 83753.06938368

If you suffer a worse arrangement, the total goes down much further.

• ( 100000 - 4000 ) * ( 1 + -7 ) = 89280
• ( 89280 - 4000 ) * ( 1 + -4 ) = 81868.8
• ( 81868.8 - 4000 ) * ( 1 + -3 ) = 75532.736
• ( 75532.736 - 4000 ) * ( 1 + 0 ) = 71532.736
• ( 71532.736 - 4000 ) * ( 1 + 0 ) = 67532.736
• ( 67532.736 - 4000 ) * ( 1 + 7 ) = 67980.02752
• ( 67980.02752 - 4000 ) * ( 1 + 7 ) = 68458.6294464
• ( 68458.6294464 - 4000 ) * ( 1 + 20 ) = 77350.35533568