High performance DSP processor with better on-chip memory I am currently using a combination of 4 ADSP 21469 because of the complexity and huge chunk of code that I currently have. BOOT and application will execute from external Flash memory. The code for radar processing are equally distributed across 4 DSPs

Even after splitting the code across 4 DSPs, I still need to perform code optimization because of memory constraint. By doing a certain level of optimization, my variables are not accessible due to which I have to read from the registers, which vary all the time. 

My thought was to use a single advanced processor that is robust enough to contain all the code so I can use common libraries wherever I can to make my code efficient on target. 

To summarize, the bottleneck is that ADSP 21469 has 5 Mbits of on-chip RAM, 4 Mbits of on-chip ROM which is way less than my requirement. If I can use a multicore processor(Ex:quad code) that works at >=400 Mhz, which has an on-chip memory of >=20 Mb to hold all my data fr a faster access or supporting external memory without being slower in accessing the data.
That depends a bit on the constraints: are you out of code space or data space or both?

Anyway, the typical way of dealing with space problems is to use external memory in conjunction with a fast access internal cache. An alternative would be something like this http://www.ti.com/lsds/ti/processors/dsp/c6000_dsp/c66x/overview.page with a few gigs of DDR attached.

Obviously external memory is somewhat slower than internal memory, so the trick is to move data efficiently into internal memory, so that it's there when you need it. In many cases the processor will do a pretty decent job in managing this automatically, especially if it's mostly code (not data) and most MIPS consumption is in loops. 

If that's not good enough, there are certainly strategies to manually optimize code for cache efficiency as well.