When I last wrote about Shift4 in March, shares were trading around $44 and the sentiment around the stock was awful.Management had just put out 2026 guidance that came in well below what the Street expected. The old $1 billion free cash flow target disappeared from the presentation. Global Blue was dragging on margins and cash conversion. And after years of questionable communication, the market basically decided it was done giving this company the benefit of the doubt.I understood the frustration. I was frustrated too.Then the stock went on to fall another 20%.FOUR eventually traded down to $34 in June before bouncing back above $50. From the low, the stock is now up nearly ~50% in a little over a month. After watching it get pummeled for most of the year, I will gladly take it.If you want to try Koyfin, the tool I use for all my charts and research, you can get 20% OFF with this link.But whenever a stock rallies this quickly, you start hearing the same question: did we miss it?I don’t think so.At $39, which is where I built the model below, Shift4 was trading at roughly 4.2x my estimate of 2027 free cash flow. At today’s price around $50, the multiple has increased to roughly 5.3x.I want to be very clear about the denominator here because this is where the math can get sloppy.The ~5x figure is not based on the old $1 billion free cash flow target management quietly removed earlier this year. I’m using approximately $740 million of adjusted free cash flow in 2027, with roughly 78 million diluted shares.That works out to around $9.49 of free cash flow per share. The stock is trading at just over five times that number.My estimate is actually more conservative than the $800 million to $900 million year-end run-rate I discussed in March. The $740 million represents free cash flow earned across the full year. Shift4 could exit 2027 at a higher run rate without producing $800 million or $900 million during the calendar year itself.Either way, I do not need the abandoned $1 billion target for the valuation to work.The Cash Flow RampThe whole thesis now comes down to whether cash conversion recovers as the Global Blue integration matures.Shift4 generated approximately $500 million of adjusted free cash flow in 2025. I have them producing roughly the same amount in 2026, followed by $740 million in 2027, $920 million in 2028 and just over $1 billion in 2029.That sounds like a large jump, and it is. But the assumptions underneath it aren’t especially wild.For 2027, I’m estimating:$3.15 billion of gross revenue less network fees$1.56 billion of adjusted EBITDAA 49.5% adjusted EBITDA margin47.4% free cash flow conversion$740 million of adjusted free cash flowThe conversion assumption is the most important piece. Shift4 converted roughly 51.5% of adjusted EBITDA into free cash flow in 2025. Management is guiding to only 42% in 2026 as the company works through Global Blue integration spending, heavier interest expense and some ugly working-capital timing.My 2027 estimate assumes conversion recovers to 47.4%. That would still be below the 2025 level.