Straight 4m pipe from behind the wc - ( bent pan connector) 50mm higher @ WC than T into soil stack That really is the min. with these low water flushes. should be steeper but that`s a real world scenario using one length of pipe
Minimum required under Building Regs is 18mm/metre or total 72mm for a 4m length pipe; personally I try not to go less than 25mm/metre.
I assume you are aware that installing a new W/C is controlled building work, subject to several Building Regs. should be notified & inspected (fee involved). If is to be inspected, the BI won’t pass it if the fall is less than the minimum.