Matthew is the most divergent of the four: out of all the fulfillment quotations in his gospel, only three (1:23; 3:3; 13:14-16 - all quotes from Isaiah) are Septuagintal. The rest are unique translations/paraphrases/interpretations which interestingly reveals influence from the (proto-)Masoretic text or from targumim (Jewish Aramaic translations of the OT). We're not sure if Matthew himself brought the LXX text closer to the Hebrew or whether he had employed a Greek version which has done so.
Mark predominantly uses Septuagintal quotations (1:2-3; 7:6-7; 12:10-11; 12:36), although he departs from it in a few passages (4:12, which departs from both the Hebrew and LXX). Same goes for OT allusions (4:29 "But when the fruit yields itself, immediately he sends out the sickle, for the harvest has come;" cf. Joel 3:13 MT).
Luke is mostly Septuagintal (only 7:27 diverges from the LXX text), though he technically has relatively relatively few direct quotations in his gospel; what he tends to do instead is to allude rather than quote.
John is evenly divided: in four passages where he quotes the OT, his Greek is Septuagintal (10:34; 12:13, 38; 19:24); in several others he makes some minor adaptations to suit the context (1:23; 2:17; 6:31, 45; 15:25; 19:36); in yet four others he diverges completely from the Old Greek (12:15, 40; 13:18; 19:37).