Many say that publisher's to blame and whilst i would default to that too, especialy given that SR3 and 4 were both one of games that were meant to save dying publishing houses (and both failed because of same reasons Blood 2 was disaster - unfinished games rushed and studios pushed before they had time to develop more and more.
it is sad how glorious SR2 was and how much blundering there was with 3rd... offcourse 4th was just conceptualy bad and was not ever going to even aspire to SR2 greatness - it was meant to be datadisc for third afterall. It still bore the name and nnumber of the series.
Shamefull decline and sad story of once greatly respectable gem of a game.
If someone did remake (like Mafia 1 gotten in last few years) that could have been incredible! tho there are many holes to fill left - lots of story needs to be written so i fear noone will ever give it the treatement it'd deserve.