As previously stated by GavinAsford and Lucian you will need to run 4 x Cables with 4 x Antennas in order to achieve true 4x4 Mimo,, you cant just magic something up using 1 or 2 Cables with T Pieces or Splitters its unlikely to perform as well.
If you really are stuck with mounting indoors near a window, try and get the Antenna in the Upper middle part of the Glass Window clear of the Window Frames (most Double Glazing Frames have metal inserts to protect and seal the outer edges of each Glass Panel).
I have done something simular for someone using 2 lots of Huawei Indoor 4/5G Antennas (2x2 Mimo each) and mounted them approx 1 meter apart with one Antenna pointing directly at the Mast Area and the other Antenna slightly offset to it in order to try a pick up a Reflected Signal on one of the Mimo Pairs.
Another option is Poynting do a 4x4 Mimo 4/5G Panel that can be Mast or Wall Mounted but also comes with Window Suction Caps for Window Glass mounting, Ive actually got one these but have yet to test it.
Your also need a relatively decent Signal in order to take full advantage of the 4x4 Mimo Signal path over normal 2x2 but the Speed benefits are there if achievable.
Out of interest which Router are you planning on using as most do not have external 4x4 Mimo Antenna Ports normally just 2x2 and sometimes the External Ports do not cover All the required Bands for 4/5G