On a stock 4-layer boards, the pre-peg is usually quite thick, so laying 50-Ohm traces on the top layer is impractical - they would have to be too wide.
This is not necessarily the case. Infact, if you order 1.2 mm thick 4 layer boards from PCBWay, they will use 0.13 mm prepreg (full stackup is 0.035 Cu/0.13 PP/0.035 Cu/0.73 Core/0.035 Cu/0.13 PP/0.035 Cu), which means 50 Ohm traces are quite narrow at ~0.225 mm. That is what I've done for my board with HyperRAM chips as they operate at 166 MHz DDR (so DQ lines switch at 333 MHz), and it works with no problems on a real board. Of course in breakout areas I had to use minimal-width 0.15 mm traces, but this is universally recommended practice as there is simply no other way to do it.
Currently I'm trying to see if I can route FPGA + DDR3 on a 4-layer board with 0.125/0.125/0.25 process, already attempted a couple times, but failed
But now I'm much better quipped to do that tools-wise, so I'm hopeful I would be able to get it done and the board will actually work