So you do have dark fiber between the sites? There's a passive 80Gbps to 8x 10Gbps splitter solution and it's called CWDM. Mikrotik offers some CWDM gear. You have to use CWDM SFP+ modules (instead of normal 1310nm SMF modules), a passive
MUX which joins/separates different wavelengths at each end of link. There are 10Gbps SFP modules for CWDM (Mikrtotik offers 1Gbps modules with model names S-C
xxDLC40D, other vendors, like
FS.com support 10Gbps CWDM modules). So for your particular use case, you would need 4 pairs of CWDM modules (each pair using distinct wavelength) for 4x 10Gbps links and a pair of MUXes (one for each end).
Yes, SFP modules are a bit pricier than normal 1310nm modules, but if use case demands it, it's possible (and feasible).