A Pentium 4 is just too slow - the official min CPU is a core 2 duo
The PC version of GTA 4 has CPU power for breakfast - independent on clock speed, L2 cache or number of cores. The latter ones are very important, because without quad-core power GTA 4 is noticeably less enjoyable. For example: A Q6600 is about 52 percent faster than the dual-core E6600 when both CPUs are running at 2.4 GHz. If the clock speed is increased, the difference gets smaller, but is still huge and a QX6850 is 47 percent faster than an E6850. Dual-core processors have a serious disadvantage and only with 3.6 GHz an E8500 is able to beat the Q6600, but GTA 4 nevertheless suffers from frequent lags on dual-core systems - no matter if an AMD or Intel CPU is used. The Phenom X4 9950 easily passes the Q6600 and theoretically is on the same level as the Q6700 (not included in the benchmark).