Trapped myself Harmonically

I'm writing Dies Irae and I've basically been using the same 4 chords (all of which are minor) and now I'm trying to write my Quantus tremor and I'm just stuck because I've used my 4 Chords so much already that it'd sound repetitive if I used them again. 

Which style are you trying to emulate (if any)? There are a number of different solutions, the most common one being the V/iv (I) so you can set up a chain through the circle of fifths and hop off whenever you want to. The other way is to use the deceptive cadence (less popular) to set up sequences in your melody. I generally notice a lack of the III chord in the piece, so that could be a good place to go to to set up relative relationships. 

Some ideas to modulate to unrelated tonalities:

From the last chord (A) you can go to Bm and stay in that tonality.

You can consider that A cord as bIII of F#m:   D - C# - F#m....

Put an Adim7 after A to fill the measure and use it as dominant to anothe tonality, for example  Adim7 (=F#dim7) - Gm (or Gmaj).

Having the A chord in first inversion, you could consider the neapolitan sixth chord of another tonality: N6 (A) - Ab/Eb - Eb - Ab.