Using four SP relays is not a safe thing because you can't depend on all of them to have exactly the same switching times. If one relay is "slow" you could end up shorting the power supply. Much safer to use a DPDT relay to control direction and a SPST relay (or transistor) to control ON/OFF (or speed if using a transistor and PWM).
Actually, it would be best to use two SPDT relays, with the lever of each connected to the motor, and the NO contacts connected to +V and the NC contacts connected to GND. This way, you could switch either one to control direction, and both off (or on) for brake. A couple of Bosch 20A 12VDC automotive relays would fit the bill perfectly, and can probably be found fairly easily at a junkyard for a few dollars each. They are designed for these kinds of loads (ie, windshield wipers, pumps, windows, A/C fans, etc), and so are engineered to take the abuse. Even brand-new they aren't that expensive.