In afternoon trade, the S&P/ASX 200 Index (ASX: XJO) is back on form and charging higher. At the time of writing, the benchmark index is up 0.9% to 8,539.7 points.
Four ASX shares that have failed to follow the market higher today are listed below. Here's why they are dropping:

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4DMedical Ltd (ASX: 4DX)
The 4DMedical share price is down 2.5% to $5.50. Once again, this appears to have been driven by profit-taking from some investors. Investors have been bidding this respiratory imaging technology company's shares higher this month after it made a big announcement. 4DMedical advised that its CT:VQ technology has been deployed at the Mayo Clinic in the United States. The company's managing director and CEO, Andreas Fouras, was very pleased with the news. He said: "Mayo's deployment is uniquely significant. When the world's number one hospital chooses to use your technology, it sends the strongest possible signal to the entire U.S. healthcare market about the clinical value and readiness of CT:VQ." 4DMedical shares remain up approximately 30% since this time last month.
New Hope Corporation Ltd (ASX: NHC)
The New Hope share price is down 5% to $5.81. This has been driven by the coal miner's shares going ex-dividend on Tuesday. New Hope recently released its half-year results and declared a fully franked interim dividend of 10 cents per share. Eligible shareholders can now look forward to receiving this next month on 20 April.
Santos Ltd (ASX: STO)
The Santos share price is down 1.5% to $7.94. This may be due to reports that Donald Trump is intent on ending the US-Iran war sooner rather than later. Traders may believe that this could mean oil prices will pull back from recent highs.
St George Mining Ltd (ASX: SGQ)
The St George Mining share price is down over 4% to 11 cents. This is despite the rare earths company announcing a memorandum of understanding with Tecnicas Reunidas. The two parties will work to complete processing test work on samples of the rare earths resource at the Araxa niobium-REE Project in Minas Gerais, Brazil. St George Mining's executive chairman, John Prineas, commented: "We are very excited to be working with Tecnicas Reunidas to further assess the optimal processing route for the Araxa rare earths and to potentially access the vast European market for rare earths."