First, a miscalculation on your part - a 36 Hp can supply two steam turbines worth of steam, not one. So you're looking at 1500-1600 EU/t from those four boilers, not 800.
Second - Biofuel. You can fuel additional boilers by converting the saplings from your tree farm into liquid fuel. There are two options for this: MFR biofuel, and Forestry ethanol. The former can be much more powerful, but it requires you to run the biofuel processor with up to 6 different kinds of plant matter at once. If you have only one kind of sapling but immense masses of it, then Forestry is likely to be more efficient.
Third - Tree breeding. Depending on what tree you currently use and what else you have available to work with (and whether or not MFR is already capable of growing Forestry trees in 1.5.x, I forget) you can massively increase the output of a tree farm in both charcoal and ethanol by getting
some new species. This becomes even more pronounced if you then breed a hybrid with good traits. Though compared to just getting the species, which is easy, the hybrid breeding takes real work. That of course means you're going to need even more steam turbines...
Fourth - Tower of Power(tm). Look for a cheap to build, moderate-efficiency nuclear reactor design with 4 chambers or less, and then stack dozens of them. Try for something around efficiency 4; running at 3 will consume uranium too quickly. Also look into running a breeder to multiply your uranium yield. Also, if you are brave, you can look into CRS reactors. Using additional fuel-less reactors as external cooling towers and fully automated coolant cell cycling, they can run at higher efficiencies and output than regular, internally-cooled designs.
Fifth - Mix and match. Don't look for a single source, but rather use multiple different ones. Combine reactors and steam turbnes, and if your base wasn't underground I'd have said throw solars into the mix too. You could also set up a nether pump to supply a large bank of geothermals; fifty of them will produce another 1000 EU/t.
Finally - Gradual buildup. Don't look for 10k EU/t at once, but rather start with 2k and, once that is up and running and powering a drill and/or massfab, use the gain from that infrastructure to expand it.