Do you want to program your own sounds with synths/samplers.
This vid will show you what kind of things can be done with a nice sampler [asr10]:
In that vid it's used as a sampling synthesizer.. maybe you just want something that has preset sounds, that load instantly, in hiphop currently motifs & microkorgs are in fashion, personally I hate them.
Search around on youtube, study your fav artists and any sounds they use that you like, find out what gear they used,,, etc
I use a mixture of techniques for creating songs, some of my samplers I use just to record audio phrases from various synths/other samplers that I play live, then trigger those sampled phrases from my mpc, drum patterns I do often with the mpc, but it is usually triggering drum sounds I have in other samplers via midi, or it's sync'd to my sp12, which I program the drums on.
Then I have some syths which are midi'd up to my mpc, all sound sources from the synths and samplers go into my mixing desk. Then I record the 2 mix outputs from the desk into my PC/sampler..
What other people do alot of, is they track out all their audio, like in my case I could have each audio channel on the desk recorded into my PC, and then add vocals, edit/create the arrangement etc...
Personally I don't like that way, but you have to experiment and find out what your like and what works for you.
Your other option is to sell the mpc4k, buy something like the renaissance/maschine, and have all your sounds coming out of your PC.. If you don't mind sitting in front of a PC to make music, it might be a nice option.