April is stated on official bussines site of SCEE (teh link comes from there), and GT4 is an new entry on the list.
Unfortunatelly, on the full release list of SCEE, GT4 is still mentioned as Q2 (second quarter) of 2004. release, so there is a discrepancy. Intresting is that GT4 still have no official publisher mentioned: sweet sentence is saying "to be confirmed".
More unfortunatelly, on the press-given official release list of SCEE (12. february, AKA yesterday), only one game is listed for April release: Rise of Honor, together with release of stand-alone EyeToy camera. GT4 is not mentioned there, and it's not mentioned either in Q2 (SCEE games that will be released in that period are Sing Star and Syphon Filter: Omega Strain). Notice that list I'm talking about is the list for European releases-only.
There is a possibility that info about April release have com elater yesterday, but we'll se that in next few days.
My oppinion is that GT4 will be released after the E3 fair in May, but that's just IMHO.