For careful movement when dealing with ice, it says "alternatively, players who do not wish to pay this cost can announce their intention to pass through the terrain at full speed before moving their units. After the unit moves 1 hex, the player must make a PSR/DSR.
If the unit is a 'mech and the roll fails, the 'mech immediately falls and must stand up per standard rules before continuing the movement (if this fall occurs on pavement, the unit skids and its move is over). If the roll succeeds, the 'mech remains upright and the unit may move per standard rules. However, the player must make an additional PSR/DSR for each non-clear hex and level change through which the unit passes.
If the unit is a vehicle and the roll fails, the effects depend on the specific terrain and conditions. On icy terrain, the vehicle skids."
For ice, Mechs and non- hover ground vehicles: every time a unit enters a hex not using careful movement it must make a PSR with a +0 modifier; if a PSR is forced on a unit in such a hex (due to damage, skidding, charges, etc.), apply a +4 modifier to the PSR.
I hope this clarifies when the +4 mod comes into play.