dt <- as.data.frame(rbind(c("m", 1.75,  -2, 8),
                          c("m", 1.85,  -1, 10),
                          c("m", 1.65,  -3, 12),
                          c("m", 1.95,  -2, 11),
                          c("f", 1.75,  2, 10),
                          c("f", 1.85,  1, 11),
                          c("f", 1.65,  3, 8),
                          c("f", 1.95,  2, 20),
                          c("u", 1.75,  -2, 1),
                          c("u", 1.85,  -1, 17),
                          c("u", 1.65,  -3, 9),
                          c("u", 1.95,  -2, 16)
                          ))
colnames(dt) <- c("x4", "x1", "x3", "x2")
dt$x4 <- as.factor(dt$x4)
dt$x1 <- as.numeric(dt$x1)
dt$x2 <- as.numeric(dt$x2)
dt$x3 <- as.factor(dt$x3)
summary(dt)

model <- lm(x1 ~ x3 + x4 + x2, data = dt)
model

alias(model)

Model :
x1 ~ x3 + x4 + x2

Complete :
    (Intercept) x3-1 x32 x3-2 x33 x3-3 x4m x2
x4u  0           1    0   1    0   1   -1   0