You can concatenate the variable into the string as a literal, as I posted in the comment:
engine.eval("tmp<-c(" + x + ", 4)");
This works because (I'm assuming) the engine needs to evaluate literal expressions (with number values instead of variable), and the above expression essentially passes tmp<-c(7, 4)
through concatenation (combination) of the strings and integer value. I would try also first running a command to store a variable and then reference it, ie:
engine.eval(x <- 7);
Then try your original expression. I'm not familiar with Renjin, though, so it's a bit of a shot in the dark.