That is a tough question but I shall try. The C4 is capable of warmer sounds than the buzzy sounds you refer to but nothing like what could be mistaken for a mellotron or strings*. Using the sine wave, monophonic or polyphonic pitch or dry inputs, the envelopes, harmonizer and other effects can result in a wide range of very pleasant textures that are nearly impossible to create with any other pedal on the market. The only exception might be the Panda MIDI Future Impact.
The Ultrawave is a toolbox of sounds with a panorama of distortion sounds and textures and ways to control and modify those sounds and enhance your bass with a preamp, compressor and EQ. Paired with the Spectrum you will have a powerful tool for creating synth sounds and would not be missing the C4 much sound wise.
*EHX
Synth9 and
Mel9 may be more what you are looking for when it comes to strings and Mellotron sounds. I have both of these in my collection too and use them regularly. The Mel9 has all the familiar sounds of the Mellotron including the graininess and loop sounds and track well with accurate articulation, I use all the sounds one time or another. The Synth9 is a bit different as it has a bit more specialized sounds that are difficult to get otherwise I find myself using the Arp Strings the most but do occasionally use some of the other sounds.