@William ********
you'd have to BUY a single entry tourist visa from the thai consulate in Lao while you're there, apply online, go to the consulate to pay, then wait for the visa to be approved..
you said you only want 4 months here, so just take the 90 from the tourist visa and one extension, then use a service to bounce you out/back at the land border to get a free stamp 30 day visa exempt entry