I take it the code that you posted does not have the settings that you used -- your original post said you sample at 1MHz and record for 4 seconds, so I'm assuming Rate = 1000000 and Number of Samples = 4000000, correct? Or is there a loop somewhere we are not seeing?
Express VIs are fine for LabVIEW "Sales Reps" who are trying to show students how "easy it is" to write LabVIEW code (they are, in fact, correct -- LabVIEW is quite easy and fun, but it goes much better if you use Real Tools and not Crutches), but should (largely) be reserved for "toy" or "demo" routines, not for doing real work. Instead, learn LabVIEW, learn how DAQmx works, learn how LabVIEW lets you do two (or more!) things at once (like acquire data while streaming it to disk).