The A4000 motherboard isn't clocked off the CPU card. If you have an A3630 card, that's clocked off the motherboard, so you can't overclock it without overclocking the whole system. This isn't true of the A3640, that's independently clocked. The problem with overclocking the A3640 is that you are also overclocking the glue logic on the card. You'd have to be very lucky to get a card that will tolerate a 40MHz clock.
The slowest device on the A3640 (except for the 040 Cpu) is a 15 ns Pal or Gal chip (most A3640's have 10 ns or faster) and these are rated by their manufacturers to run at 50 Mhz or above. However, the Amiga custom chips on an A3000 or A4000 motherboard were designed to run at a mere 28 Mhz! So you're lucky if you can overclock your motherboard at 33-35 Mhz. Also, Ramsey is highly dependant on the speed of the installed Fast memory. Also, The A4000 must receive it's Cpu clock and Clock 90 from the Cpu card in order to support the A3640.
