Collective sigh of relief, Park Slope: Word came in from a reader that “no more trips up to the Upper West Side for comfy if expensive sheets” are necessary because the fourth outpost of Laytner’s Linen & Home is opening on Union Street between 6th and 7th avenues. And so it is. The banner outside the future store says it’s slated to be business sometime this fall. The interior of the space is looking mighty raw right now, but workers were busy trucking in building materials yesterday. GMAP